To understand what you do. It is worthwhile understanding more than what you do.
To expand on this a little bit...
Even if you have no interest in ever seriously taking up a second art, it's worthwhile to occasionally explore different systems just to give you additional perspective into your primary martial art. There's an old saying that "a fish doesn't know it's wet." When you've only ever trained one system, it's common to have a lot of unquestioned assumptions about how things are done, just because you've never seen them done differently.
When you explore a different system, you may encounter different body mechanics, tactics, and training methods. Your first goal should be to figure out
why they do it that way. Once you've started to understand this different approach and the advantages it gives, you can ask yourself why it's done differently in your primary art. Once you see more options for how something can be done, you are in a better place to evaluate the advantages and disadvantages of each approach. That can lead you into a whole journey of discovery into how the different components of your art (principles, mechanics, tactics, techniques, and training methods) fit together.
Perhaps you study art A and spend a little time exchanging knowledge with an expert in art B. You find that art B has a way of throwing a punch which is more powerful than what you have learned in art A. So why doesn't art A use this punching method? Maybe it's because that method leaves you more open for a certain type of counter. Why doesn't art B worry about that counter? Because they have a certain game plan for when that counter comes? Why doesn't art A use that game plan? Because art A is built around a certain set of tactical priorities for a certain context and that game plan would violate those priorities. Once you see that art B is built around a different set of priorities for a different context, you begin to become more aware of the foundations your own art is built around. You are the fish who is starting to understand water.
At other times the new art might use some of the same principles as your own, but they are presented or practiced or conceptualized differently. Sometimes seeing that different explanation can give you a breakthrough in understanding how those principles are applied in your primary art.
Another possibility is that exploring a new system will make you aware of the limitations of your primary art. That doesn't necessarily mean your primary art is bad or that it needs to be changed or your need to study additional arts to "round yourself out." Every system has limits. Every combination of systems has limits. Every training method has limits. Every person has limits. That's reality. This being the case, in the words of Harry Callahan, "
a man's got to know his limitations." When you know what you don't know, then you can adjust your game plan accordingly. When you think your system gives you something it doesn't ... let's just say that the Dunning-Kruger effect can lead to painful outcomes.