well, if you want to create a group you need to know if the player u're taking is good or not, and you have NO way of knowing that without playing with them in the past, or some1 vouching for him (which is also a risk).
So... people hide behind iLvl, thinking "oh, this player has 845, he's been to mythic, so if some1 already took him, and his ilvl is what shows it, he's good enough for me"...
I myself find it funny that mostly, the peopel creating the groups and asking for 840+, are not 840+