I've been replaying through Undertale for the first time since its release and during the Omega Flowey fight I noticed he was able to use multiple save slots even though the game only has one. I wondered why this was, then I realized the highest number slot he uses is 6, so my theory is that the number of save slots he gets is determined by the number of souls in his possession.

If this same logic applies to Deltarune, then there's actually a specific reason we get to have 3 save slots instead of one, and that reason is that we have 3 main party members that we have control of as the player, each possessing a soul, assuming darkners have souls.