turret defense sections in Nova Prospekt for example. In the first encounter, you are faced with a relatively easy attack in a fairly defensible position. This makes it easy for the player to set up and monitor the turrets without feeling too pressured. Near the end of that first encounter, once you've gotten the hang of it, manhacks are sent in to force you to learn to simply trust the turrets to do their work while you deal with external threats.
Now take a look at the second encounter involving turrets. Having taught the player how to set a good defense with them, Valve clearly felt free to up the challenge and require more out of the player. Rather than giving you a single easily defensible position, they provide an entire floor with plenty of rooms for the player to choose from and take advantage of. However, none of these locations are quite as easy to defend as the command station where the first attack took place at. This greater freedom both allows the player more freedom to try out his new ability and the Combine forces more space in which to execute their attack. Increased challenge is also provided by a greater number of enemies, easier openings for grenades, (which are the ultimate threat to a turret defense) and Alyx's constant reports of more and more soldiers, which make for a significantly greater feeling of tension.
The third and final encounter involving turrets is, arguably, easier than the second, but it does something the other two didn't. If the first turret section's purpose was to teach the player and the second one's was to demand mastery of the ability from the player, then the third turret defense's point is integration. The best strategy for the second segment was for the player to hole up in a room and let the turrets do the dirty work, while watching for grenades and keeping an eye on them to make sure they didn't get knocked over. In the final one, players have to run about the enviroment, which features multiple points of entry, and help the turrets in the fight. The turret defense is merely one part of the player's arsenal in this instance.