Selecting fill puts an icon next to your given role
So your team knows you're willing to fill any lane.
This way if (because i've seen it glob knows how many times) you picked Mid>Top and I went fill and got Mid (ha yea right) you'd know that I'm willing to give you your more desired role..
Or even if it showed your other section would be nice in general. Its confusing as to why it happens but plenty of times i go Top>Mid and get Mid and someone else went Mid>Top and got Top. Why???