Zombicide: Why do zombies split?
Actually, if you have 8 walkers in one zone and there are 2 valid paths to get to the most noisy zone, you literally split them : you get 2 groups of 4 walkers.

There can only be one zombie added when splitting: in the case you have an even number of zombies. Eg: If you have 9 zombies in one zone, 5 go one way (A), 4 go the other way (B) + an additional one (B)

You apply that rule for each type of zombies (except Abominations that never split).

The idea is for the zombie turn to be as simple as possible to get it over with as quickly as possible. It is not the interesting part of the game, the players' turn are. Deciding randomly where each zombie goes would slow down the game without adding much. Adding the extra zombie when splitting even groups, makes things straight forward. I think it is a purely mechanical (opposed to themed) driven rule.