Monster Musume / Daily Life with Monster Girl - What is the evolutionary benefit of spider-butt?

Dat ass…And the other ass…and the stomach after that ass…Spider girls are weird

It's pretty implied that she had one at the usual human spot since she straddles darling when she wants the d. Her extra art also states that the pendipalps might be used for holding a mate in place.

Somewhat unrelated but I believe most fans also theorize that the human buttcheeks are muscle for the pendipalps.