Good question!  I wondered the same thing until I went to a swim clinic last week.

The instructor said you shouldn’t really go to the pool just to swim laps.  You should always incorporate drills into your workouts or you’re just being counter productive. Plus drills make the swimming less monotonous and more fun.

Examples of drills:

• 6-kicks on your side then rotate (this helps with your extension)
• hypoxic swimming (for advanced swimmers - when you  increase the amount of time between breaths by only breathe every 5th, 7th, or 9th stroke 
• sculling - using hands and forearms to isolate catch
• drafting - for open water practice try swimming just behind another swimmer
• kicking - get a kick board and practice kicking from your hip not your knee and keep your ankle loose
• 1-arm swimming

Advanced Drills:

• powerstroke - focus on hard pull and long extension
• catch up - develops good reach (forward extension) & good catch mechanics
• finger tip drag - when you lift up your arm out of the water to take a stroke, drag your fingertips along the surface to work on high elbow recovery
• swim with a closed fist - helps you learn maximize pull by using your forearm

A basic workout should include:

• warm up (15-20% of workout)
• kick set (10% of workout)
• drill sets for technique (10-20%)
• main sets focusing on speed, endurance, threshold (40-60%)
• more drills or skills (5-10%)
• cool down (50-100 yards)

Example workouts:

1. 2000 yd workout

• 5 X 100 easy on 30 sec rest between each 100
• 1 min rest
• 4 X 50 kick with 20 sec rest between each 50
• rest 1 min
• 8 X 50 drills with 20 sec rest between each 50
• rest 1 min
• 8 X 100 (1 easy:1 harder) with 20-30 sec rest between each 100
• rest 1 min
• 100 yd cool down

2.  Beginner Level (wanting to be able to swim 450-500 meters non-stop like in a race)

• 3 X 100 easy warm up on 30 sec rest between
• 100 easy w/ kick board
• 4 X 50 drills w/ 20 sec rest between
• rest 1-2 minutes
• 400-500 meters straight, no pausing at walls
• rest 1 minute
• 2 X 50 easy cool down w/ 20 sec rest between

3.  2000 yard workout

• warm up - 4 X 100 easy w/ 20 sec rest between
• drills/technique - 6 X 50 drills w/ 20 sec rest between
• kick set - 4 X 50 w/ kick board w/ 20 sec rest between
• muscular endurance - 5 X 200 moderate hard (85% effort) with 30 sec rest between
• cool down - 100 easy

These are some example workouts that were in the book I got at the clinic.  What I like about them is that they provide a frame from which you can modify to your needs.  

I hope this helps!