In case you haven't noticed, I'm a pretty big fan of mindfulness. If you're completely unfamiliar with mindfulness, I've provided a short description of what it is along with some resources for practicing on your own here. Basically, it can be defined as opening up to and more fully experiencing the present moment, just for the sake of experiencing it.
If that sounds simple, it's because it is. It's an incredibly basic and accessible skill that all people are capable of on some level. With that said, the idea of sitting still and just experiencing the present moment can be confusing and foreign at first. Many times when people are first introduced to mindfulness, their first response is "But what's the point? What is it supposed to do??" The issue here is that the human mind is a work horse. It doesn't want to sit idle, it wants to think and problem solve and investigate, and when it gets working on something it wants tangible results. As far as the mind is concerned, sitting still for 15 minutes just to notice what's going on around you is unacceptable. In my experience, it doesn't help to argue with the mind about this one. Trying to come up with good reasons to convince yourself to practice mindfulness is often met with equally good reasons to not practice (e.g. "I've got a million things to do, I don't have time to sit around and do nothing for 15 minutes."). Instead, I find it more helpful to step back and question whether a reason is needed in the first place. There are other things that we do as human beings that are very similar to mindfulness, but don't require good reasons, let's take a look at one.
Have you ever known or been around someone who might be considered a "connoisseur?" Maybe you've seen a wine connoisseur or a cheese connoisseur sampling some wine or cheese, or maybe you know someone who has a real passion for a particular type of food or beverage (e.g. chocolate, bourbon, Indian food, etc.). When you observe a connoisseur consuming the product they are passionate about, they do it in a very particular way. For example, wine connoisseurs like to swirl their wine around to study it visually, stick their noses down into the glass to smell it, and once they finally get around to sipping it, they proceed to slurp and gargle the wine inside their mouth in a way that can be fairly disturbing if you're not familiar with it. Here's the interesting thing about all of these behaviors: they are all done simply for the sake of experiencing the wine more fully, and that's it. Of course there are sometimes ulterior motives involved if the person is being paid to do this or is testing a wine to sell in their restaurant. But bottom line, it's about pausing, opening their senses up fully and experiencing every noticeable detail about that glass of wine. No justification required.
So what if we extended the idea of being a "connoisseur" a bit further, into areas where we wouldn't typically use the term connoisseur? Consider someone who absolutely loves gardening and spends all of their free time tending to their garden, closely examining the plants, taking in the scent of the flowers and vegetation, carefully feeling and studying the leaves and stems and soil. We wouldn't think to use the term with this particular hobby, but these are the behaviors of a connoisseur. And the same could be said about many other hobbies and interests.
Now let's take it one step further. Let's imagine a person who was a connoisseur of present moment experiences, who approached all of life's experiences with this connoisseur-like behavior of pausing, opening up fully to their present moment experience, and soaking it up just for sake of experiencing it. Well, that's mindfulness! And the sense of presence, and enjoyment, and total engagement that a connoisseur gets while experiencing the thing they are passionate about... that is the justification for practicing it. Might it be acceptable for you to sit and "do nothing" practicing mindfulness regularly if you just considered yourself a "Present Moment Connoisseur (PMC)" in training?
Typically mindfulness starts with sitting quietly and closely observing and experiencing things like your breath, your bodily sensations, and the sounds going on around you. Yet, those who practice it regularly will notice that this skill begins to seep out into their day to day experiences. Suddenly you become more aware of everyday things that seemed unimportant before, or that you just simply didn't take the time to notice. The smell and feel of clean sheets on your skin, the comfort of your morning coffee, the weather (any kind of weather), the feel of grass beneath your feet, the feel of your toothbrush on your teeth, the texture and flavor of the food you eat, the physical presence of a loved one, and on and on. And with time, you not only notice these things more often but begin to actively seek them out and amplify them in order to experience them more fully, like a wine connoisseur seeking out and studying new wines. With time, you become a connoisseur of the present moment.
So if mindfulness interests you, but your mind (like most minds) is demanding reasons and justification for why you should waste your time sitting still and "doing nothing," then ask yourself this: Do I really need my mind's permission in order to simply pause and experience the present moment more fully? Does experiencing the present moment require a good reason? A connoisseur doesn't have to explain themselves when they dive into and carefully study the thing they are passionate about. And as a PMC in training, neither do you.