There are several reasons to practice catch and release.  Releasing a fish allows it to be caught again and be enjoyed by other anglers.  Also, fishing regulations often require fish of a certain size to be released.  Stress on fish increases when being caught, landed, and released and must be minimized in order to survive this process.

Stress is negative, whether you’re a person or a fish, and the higher the stress level, the longer it takes to recover.  When fish are caught, a number of factors contribute to how much stress they experience. Effects of stressors on fish add up quickly.  The more stress they experience, the lower the chance they will survive if released. 

Just because a fish swims away does not mean it will survive.  Fish that have been badly stressed may die a few days after being released with out being seen.

Ways to Reduce Stress and Increase Fish Survival
Answers to Commonly Asked Questions
Rating Common Fishing Techniques for Impact on a Fish's Chance of Survival if Released

Rating Common Fishing Techniques for Impact on a Fish’s Chance of Survival if Released

Minimal Impact

Moderate Impact

Substantial Impact