Honestly, its because some of us want to have a real conversation about the music. When stuff isn't so popular, you know that anyone who actually knows the artist must be a pretty serious fan. Once its popular, you get a whole lot of "OMG that group is liiieekk sooo good!!" and that just gets annoying as shit honestly. So even though it may seem meaningless to know if someone knew about a group before mainstream success, it is simply a way of finding people who are actually worth talking to about music. Sure, its not a perfect system, but it helps I guess...