When people are busy with school, work, extracurricular activities, and keeping a roof over their heads, they might not have time for much else. Consequently, they might wonder what people are doing being reclusive and pondering their existence. After all, why do people think about the meaning of life instead of simply living? Well, there are two reasons: planning and motivation.
In the former reason, a person focuses on meaning when they are making life plans and decisions vs. executing, taking action, and striving for attainment of those goals. You can’t attain a goal that you don’t (know you) have. So some people spend time thinking about how to approach their lives, and others focus on living using that approach. If you want to achieve major goals in life, you could just start doing something and seeing what works, but you also could make much more progress if you know what goal you were striving for ahead of time and could think about and plan for the best approach. If there is a way to know how to approach life better, you should ponder it; if the only way for you to find out about something is to experience it, you should experiment.
In the latter reason, we're now talking about someone spending time pondering why he or she exists instead of going about his or her day or even just living life "in the moment" and just being. There is one simple response to that: people don’t see the point of living if they don’t find meaning in it. For example, you might love going to the beach and enjoying the sun, sand, and surf, but someone else might ask themselves, “Why go? You’re just sitting there outside in the sun getting skin cancer and doing nothing? Seems pointless.” It’s that questioning of the purpose, motivation, or point that gets people to ask themselves about the meaning in their lives vs. living.
People need to believe that what they are doing leads to one of three things: it makes their life or others' lives better in some way, it is significant in some way (usually due to making someone's life better), or that it makes them feel happy and fulfilled (usually due to making their lives better or being significant). This is why depressed people don't see the point of anything: they don't see how anything in their lives will get better; it won't seem to have a point or significance; and it won't make them feel any better. On the other hand, happy people believe in a positive future, believe that what they're doing is meaningful to them, and believe that what they're doing makes them feel good. So understanding why they're doing something and finding a "why" that drives them to act is necessary to live life with more meaning, purpose, and fulfillment. This is why people want to find meaning in what they're doing instead of simply "mindlessly living" from day to day.
Now, there is one important distinction between mindlessly living and intentionally living: mindless living is often a result of having no purpose and usually results in pleasure-seeking behavior, whereas intentionally living is a result of finding things of value and significance to attain and maintain and results in fulfillment. For example, people who use drugs and alcohol a lot are usually seeking pleasure to cover up an emptiness or pain inside from either a loss of their previous values, significance, or purpose or a lack of it. So they’re using pleasure to replace that unfulfilled feeling. People without purpose but also without pain might instead go pleasure-seeking to get the next “life high” only to realize after a while that this type of happiness is fleeting and, ultimately, unfulfilling. In either case, living without an aim to make your life or someone else's life better in some way or doing something of significance will cause you to wonder why you're doing what you're doing—or even why you're living at all.
In those cases, you do need to take the time to ponder your meaning so that you can find what gives you fulfillment. It could be raising children, working for a cause, doing something you love like writing, painting, or playing music, or building houses. Whatever it may be, the point of pondering it is to get clear on what matters to you, what your goals are, and what you want to spend your life doing so you live every moment with just that much more intentionality. That can make all the difference in motivation and drive. If you just lived your life without knowing what made you feel fulfilled, you might just live in a “Groundhog Day” situation where you lived the same day over and over again and it didn’t feel like it mattered (I reference that movie because Bill Murray aka Phil Connors turned the exact same day from being meaningless pleasure to fulfilling through growth, self-improvement, and helping others).
So once you know what brings you meaning, you can live and not think about it too much anymore. But the depressed, lost, unfulfilled, and hopeless need to find something that gives them the thought that they are working toward a better future that matters to them, and so they do need to ponder it. “Mindlessly” living could just be ignoring a need to reevaluate your priorities and make sure that you are on the right path.
One final note on the other side of this issue: some people absolutely spend too much time in their heads thinking instead of being present. That’s why mindfulness or "being present" is such a popular practice: it balances you back out. Being present allows you to enjoy the experiences you have, avoid feeling stressed and overwhelmed, and bring more calm and clarity to your life. Therefore, if you are overthinking because you are trying to solve problems all of the time or handle life stress, living in the moment a little more would be beneficial. If you are ignoring handling an emotional issue or life situation by trying to cover it up with life highs, however, you might need to sit back and do some soul-searching instead of living day-to-day experiences.
Should you live every moment of your life to the fullest? Sure...but if you don't know what "fullest" means, you need to think about it for a while. Without living, you can't have the experiences that will lead to growth and a sense of purpose and significance. Without meaning, it can feel like nothing you do actually matters no matter how much you try to live in the moment. Therefore, you have to establish goals, purpose, direction, and values/significance to keep yourself in good spirits so that living your life feels better than it would without them.
Find meaning, and live your life. Don't live your life without finding a sense of value, purpose, significance, or meaning in it...but don't keep pondering things just to ponder them when the point of life is to live. You need to find meaning and live that purpose to feel fulfilled, so make sure that you do both!