But what are the lower-level creatures for in the game?

Did you even look at the map? Or at least the map's name? The idea of the map (as I understand it) is to have battles with super-dragons, whose lairs are scattered around the map. Accordingly, I was suggesting creating the most convenient conditions for playing specifically through the dragons. That's all.

And lower-level creatures are needed to fight with them at the start of the game, gathering resources to buy/build higher-level creatures. Isn't that logical?