Tower tactics.

As I understand it, we're talking about the so-called "intestinal respawn." It's quite an unpleasant thing, there's no denying that. The simplest example of a chain: a hero is bought, he runs, another one is bought next, he follows the first, then birds are bought, bought for the hero, and he just reaches the second, the second reaches the first. The first one defeats something, then gives the army back, and this same chain starts working in reverse – it's already built.
You might ask: but the heroes are spread out, they'll have more movement points! I'll answer: along the paths, there are often not very useful things, like piles of resources. Accordingly, we spend the "extra" movement points on picking them up.