Explain it like im 5

I put a lot of thought into whether or not I should spend time worrying about putting horses in stalls for "passive" benefits. I have concluded that without the poop mechanic, it would be worthwhile.

So- what is the REAL reason we have the poop mechanic? Don't tell me its to make the game realistic. It has to be with how much money they get when we log in or something. This is a fantasy game- so realism shouldn't factor in??