In vanilla minecraft, some mobs are undead, some are arthropods and more. The "type" of mob affects some features like enchantments. For example, undead mobs are weak against Smite and arthropods are weak against Bane of the Arthropods.
It would be cool if we were able to choose the type of a custom mob. This way custom mobs could be weak against these enchantments and would feel more realistical.
It would also be cool if we could use this in procedures: a procedure block "is <entity> a <undead/arthropod/...>.
This isn't very important since these enchantments aren't very used but I still wanted to suggest it.