With over 100 Personas to fuse in Persona 3 Reload, you're going to end up creating some just for the sake of creating something stronger or for unlocking the protagonist's Theurgy. Unlike everyone else in SEES, the protagonist has seven Theurgy Skills. One you'll want to get immediately is Scarlet Havoc, which requires you to create Siegfried and Mithras. You're likely to obtain Siegfried when you're at a higher level, but you can get Mithras somewhat early in Persona 3 Reload.