Baby Pokémon are the lowest stage of Pokémon evolution. The concept was introduced in Generation II, along with breeding. ~Bulbapedia
Baby Pokémon probably were introduced to accommodate the introduction of Breeding. It may have been considered odd to breed some Pokémon and suddenly hatch an egg to have the exact same Pokémon appear.
They are no "benefits" to baby Pokémon, except maybe better logic, something Pokémon could use more of.
Hope this makes sense
