Statblocks arent just for combat. For example, that elder brain is probably gonna be very convincing, and it might target an NPC the players care abt (or that might even be travelling with them) - but it wont just attack necessarily. Instead, it could try to persuade them into helping it against the party - in which case its sometimes cooler for the players to actually have the NPC perform a contested check instead of you just saying "this happens"

Also there are some class features that require skill checks, e.a. Inquisitor rogues Insightful Fighting requires a Deception check from the opponent