"Requires a group of 8 players" means you can join either as a full group, or if fewer, you will be merged to existing groups, if any.

The reason behind this, I can just imagine, is that the Duty Finder on raids is more appropriate on to learn the fight and experience the content, it explains the "in progress" option always turned on.

To who wants a fresh start and want more chances of clearing a turn, the more appropriate place is PF.