From what people said like literally few posts ago when I discussed them...

They are for those who got the girls in their main events, but didn't manage to "max" them a.k.a. get all skill/slots unlocked then. They can end up with 2 fully unlocked copies thanks to that.

I was buying them each time they added duplicates thinking I will just use them if those girls drop for me from gacha, but people said the event girls never are added to gachas, thus they never will drop, thus keeping those blooms is wasted FG on inventory space.