Probably to keep the doo doo inside 💩🛟💨

Think of scrunchies.

When they’re relaxed, the material scrunches up. When you expand them, the material stretches and smoothens to accommodate the expansion.

Same for your poophole, it needs to be able to expand and the skin around it needs to accommodate.

pour ye cocke stimulus

This article might have some interesting info for you:

https://en.wikipedia.org/wiki/Sphincter

So you don’t drown like a cow!

Secret third set of gripping apparati

To give you another way to move around the room. You can see dogs practice this sometimes.