Raider IO exists because players want a way to group with, or try to at least, people of similar experience as them. If you are making a 5man group and want to push a specific key you want people with similar experience as you. This you can accomplish easily with being in a guild or having a group of friends to play with, but in case you don't you want some way to know who you're playing with. If a completely clueless person joins your run that almost never did the dungeon before, you are going to have a bad experience (or the whole group will). If there wasn't Raider IO score to "judge" ppl's experience then there would be something else to replace it.