Once upon a time overkills made you receive increased exp from monsters, but this was removed a long time ago for some reason and now it’s just a fancy graphic that pops up with no effect.
Here are some suggestions to make overkills interesting / useful again:
increases the chance of monsters dropping unidentified gear
increases the chance of unidentified gears having better random and more useful stats
increases the chance of monsters dropping exp cards
increases the chance of monsters dropping monster gems
increases the chance of jackpot mobs (blue, silver, golden, red, violet shiny mobs) appearing in the same map
gives you a movement speed and physical / magical attack buff increase for some time
This could even be used for events similar to the ones where tree root crystals gave you beneficial effects, but instead it’s overkills for the next few events.
I’d like to see overkills getting some love again and not just be a useless fancy graphic forgotten by IMC.