Why circuits?

Our system of exercises can be used in a stationary way – doing several sets of the same exercise before changing to the next exercise. But the sequence of exercises is specifically designed to have you repeat an exercise for a number of reps or seconds and then immediately continue to the next one. Only a short pause to be taken between rounds. This is referred to as circuit training.

Circuits are a very time-efficient and effective alernative to stationary training methods. You use lower weights, but end up doing more reps as compared to the same sets in a stationary training and you can get higher cardiovascular responses which gets you fit.
You can read more background info of circuit training in general in e.g. this research.