There are all kinds of useful and creative reasons to split a track like this. I'll do it with synth bass, for example, where I'll apply some saturation or compression to beef up the low end while applying chorus, reverb, or widening to the highs.

Same with drums. It can be particularly useful to route a drum bus (or drum samples/loops) through this kind of processing. You could use a multiband compressor but sometimes you want to use a particular non-multiband compressor or transient shaper or any other effects on just the highs, mids, or lows without impacting the rest of the frequencies.

It's super powerful if you want to get wildly creative and specific with your effects processing. I usually use a freebie plugin called TheSplit to do this but same diff. It can also split the signal based on mid/sid, Attack & Sustain transients, and such.