Basically this. If they wanted to introduce evolutionary lines earlier in a game, they'd just give the original Pokémon next-to-useless moves until a certain level anyway. And there are plenty of "goals" to attain with any Pokémon, including non-evolving ones, such as learning a certain move or set of moves and getting it fully EV-trained.
Take a Pokémon like Budew or Elekid. Roselia and Electabuzz can't be caught until a while into the game, but a Budew or Elekid can be caught very early on. It balances some Pokémon to let you use them earlier than originally.
Pichu, Cleffa and Igglybuff existed purely for cuteness and to promote the breeding feature. Tyrogue existed to bridge the Hitmons together.
But for things like Munchlax or Azuril? No idea other than cuteness.