Suppose you play BioShock 3. You're going to expect the same old mooks to fight, and you won't have to change your strategy by an iota since BioShock 1. Right? Not if they evolve.
In the first BioShock (not the second, for some reason), Splicers and Big Daddies got stronger when you reached past the midpoint. While the BDs had more health and dealt more damage, the Splicers developed an entirely new ability. Thuggish Splicers were suddenly immune to Electro Bolt, Leadheads switched from pistols to machine guns and Houdinis started throwing ice instead of fire. Wouldn't it be cool if the enemies in the sequel started developing new powers as you progressed?
Also, new enemies!