From what I gather, it’s a mix of Will Ferrell being genuinely extremely funny and clever, having a reputation for making C-grade material funny and getting butts in seats so studios chase him to sweeten up projects, and having a reputation for being earnest and helpful and enthusiastic so he willingly throws himself into stuff.
When he’s got a say in matters, he makes delightful, odd, funny stuff. But he’s also just willing to mug on camera for a paycheck or to help a friend.