User blog comment:DeathGodPrayer/Talk About What You Want!/@comment-4547267-20131218013544/@comment-9339850-20131218014445

basically; it was an engagement ritual. Rias had broken of her previous engagement with Riser, so her parents wanted her to go through this with Issei (since they realized she loved him) to have some piece of mind that she will probably be married by the time she officially takes over as head of the Gremory Clan.