I have seen training dummies and targets all over Skyrim (and Cyridil). Do these practice targets improve your skills when you use them, or is it just for effect?
These dummies are just for effect. The Dragonborn cannot gain skillpoints from attacking training dummies, as they are not actually dealing damage to anything (which is the way to improve these skills).
Archery targets, however, are useful if you want practice lining up your shots, however, and are especially useful if you wait around until NPCs practice on them, as you can harvest the NPC's arrows from the target.
If you're bold enough to pickpocket more powerful arrows onto them, harvesting the infinite arrows from their targets is the best way to get a large supply of the most valuable arrows in the game.