You use it for the promo that has Forget Me Not on it. I ended up with Cornflower, Forget Me Not, White Tulip and Japanese Rowan so it might give just those four or it might give four random 4* Knights.
As for the 5 FG reroll, I'm torn between rolling until I get a 6* that can fill in some of the gaps I have (+defense, counters, attack debuff, first round skill proc for the squad) and rolling until I get one of the 6* Knights I like the appearance of and/or never had in the Nutaku version ('normal' Dogwood, either version of Viola, etc). Ideally I can roll a Knight that covers both areas, like Saffron or Shrine Maiden Licorice/Red Spider Lily.