The body needs recovery time when doing muscle building sets.
(Exceptions: If you are lifting as cardio then resting doesn't make much sense.)
Between sets during most workouts the rule has always been from 1.5 to 3 minutes of rest. This is not etched in stone and to each his own. Now however there is study on what to do while resting between sets.
Click through to read the study.
Bonus: Without a clock to measure your resting time remember that most songs lost 3- 3.5 minutes long. Listen to at least half of it and get back on it.