Why do cyclists ride peloton?

A large group of riders riding together in a bunch. A bunch of riders. There is often one peloton, and some escapees in front and some stragglers in the rear. Often enough there are more than one "bunch", hence more than one peloton. But "the" peloton is the largest bunch that also has the overall best cyclists in it.

In regular road racing, the riders tend to bunch up and ride in one or several groups. The main group is called the peloton.
A peloton is the "bunch", the "field" or the "pack" of cyclists that consists of the main group of cyclists in a race. The Word peloton is drawn from French where it means "ball" or, more loosely, platoon.

The "bunching up" of the riders to form the peloton is a tactic designed to save energy on a ride while remaining competitive. The riders at the front of the peloton "break the air" on the ride, and those following "draft" behind them saving as much as 40% of the "energy cost" through a given stretch of riding. A link is provided.

It is spelled "peloton" and is the main group of cyclists in a race.