Puriel

I am new to the game and trying to figure out if there is value in this membership to spend money. It says you get an 8th slot, but from what I have seen on my starter nutaku gold that doesn't mean an 8th girl on the team. Can someone explain what the 8th slot is? Also it says gives you previous Season and PoA rewards, how does this occur? The rest seems pretty straight forward. Thanks!