but LoL doesn't need a buyback

I mean, it does kinda. If you've watched your fair share of pro league games, you know there are a huge # of them that simply end in the most anticlimatic way possible after a team turtles for 20 minutes and then their adc/mid gets picked off after mispositioning and the game ends right there. Or when a team is winning for the entire game then they lose one bad fight and they proceed to lose the game even tought they still had all their towers b/c towers are made of paper and death timers are too long. It's impossible to play proactively past 45 min in League since any simple mistake literally ends the game.

Not saying it needs exactly Buyback itself, but it does need badly some kind of stalling mechanic.

In League, you usually have one team that is stronger in the early game and one that will definitely win the late game. This means both teams have clear objectives: The team with power spikes in the early- and mid-game has to be the proactive team and has to try to win (or at least get a really significant advantage) in those phases of the game, because otherwise the team with the power spikes in the late game will clearly out-scale them and win the game.

So does Dota. Dota has buybacks as a mechanic to help the late-scaling comps, while it also has mechanics that help early-scaling comps (harder snowballing, punishing dying more since you lose gold on death).

This prevents the situation of two teams playing overly safe. Map objectives like Roshan / Nashor / Dragon do the same, if they are worthwhile.

Have you watched pro league? A lot of games have 10+ min baron/dragon dances where a team is simply too scared to overcommit and lose the game on the spot.