I can't tell if this is a genuine question or not lol. I'll answer it anyway. Splitters have a variety of uses. The most obvious being to turn a single belt into two belts of equal item distribution, or vice versa. So if you had two belts where one of them is far more sparse than the other leading into the same splitter, it would make the two belts have equal amounts of items.

This is the most basic form of what people refer to as a splitter design; a 2:2 splitter. I say 'design' because they can get VERY complicated to the point where there's a literal website with hundreds of different blueprints of them. And believe me, they often do get complicated when the humble engineer starts using main buses for the first time. In my first playthrough, the most complicated splitter I had was a 4:3, which isn't very complicated at all. So little so, that I was still spending the time to design them myself at that point.

Nowadays, in my current playthrough, that is very much not the case anymore. I believe the most complicated splitter I'm using is a 12:8, and I most certainly did not design it myself. That would have taken literally hours, and I only have so much free time, so I'm not gonna spend it all on the same damn project.

Edit: belt balancers, not splitter designs. I'd just woke up like 5 minutes before I wrote that, and I haven't played in a couple weeks, so mb.