Leo is like the perfect hero for reactionary players. You literally just kind of wobble around slowly in the immediate radius around you as people attempt to fight you then realize oh fuck I'm going to lose this fight, but it doesn't matter because he has so much god damn INT now that he can throw 3 firebolts and still have enough mana for ult. You don't have to do anything, because you're the carry so the fight will ultimately come to you. You just click on the nearest shit and hope for some crits.
His win rate is super high in upper tier games. Same with Riki. Which is funny because those are both heroes that are "shit-tier" heroes (shit-tier as in, they are known for shitting on low level players, not for being bad heroes).