League of Legends (LoL) Question: why would I use overheal?

Often enough, professional games and higher ranked games have much less takedowns than your average solo queue game.
Overheal is more consistent and effectively makes the ADC tankier. It also works well with sustain supports which is one of the main purposes of the rune.

If you're in the lower elos from say Iron > Plat (maybe even low Diamond) then you can pick Triumph, because the game will just be a clown fiesta with kills everywhere you can profit on. Overheal is a typical pick if you're a high rank or if you think that the game will have much less kills.

Could be useful on junglers who have self-heal abilities as well.

I use it for Tryndamere bc he has lots of Life steal and I can use my Q at full health for bonus health