For efficient use of effects such as reverb and delay, using a send/return system is desirable. When using a reverb in series (directly on a track) the only control is the dry/wet feature. This method of processing will split your original signal, sending part of it through the effect while bypassing a percentage of that signal around the effect. Anything above 50% wet will be ultimately chewing away at the original, dry signal. When using a send and return for applying effects, you maintain the integrity of the original, dry track while simultaneously applying a secondary output of effect with the original sound.
Time-based effects are often used on return tracks while dynamic effects are placed directly onto the track themselves. To put it in basic terms, time-based effects in parallel and dynamic effects in series.