During farm events, where you farm and combine a lot of the same unit you can get a lot of exp if you use fairies. However, plat fairies are rather difficult to come by when you have 10ish plat units to feed to the main one, so using lower rarities for this is quite beneficial. I save my gold fairies from Phalanx 2 for this, and now also all the fairies from this event that i get.