There may also be a number of echelons in succession. They are independent, and (usually) ride at short distances from one other. They may be single or double echelons, with or without riders stranded in between.
For shelter and support
You can also create them when you are in a (very) large group that wants to ride together. Of course, you then keep the distance between the echelons much smaller to make optimal use of the wind. The stronger and weaker riders are divided over the various echelons in order to keep all the weaker riders sheltered.
For getting rid off the competition
In so-called echelon-stages in for example the Tour de France, they are deliberatly created to put opponents at a distance. A beautifull example is the echelon-coup of Quick Step (Giro 2017, stage 3)
A beautiful stage, with a demonstration of echelon riding in full hectics.
1. At the right time, only the QS men are in the front positions. That while everyone knew it was going to happen on that section of road and there was a continuous battle for those front positions. Only 3 others including Greipel are up there, too, all others are not. It needs to happen not too early, nor not too late, and it’s within a few seconds that all has to happen. And by all involved.
2. Determining how much space there should be so that your own men can shelter from the wind while the competitors are stuck on the side of the road and so are fully exposed to the wind.
3. Slowing down just enough by the QS men at the front so that their sprinter Gaviria can join (piloted by a team mate).
4. Those skilled proffesionals in encheloning can execute a perfect enchelon at less than full force. At full power they do make small mistakes, but they can correct them easily. For example, Jungels drives a bit too hard. He holds a bit back. Others fill the gap slowly so that the rest can flow smoothly. They have to ride closer to the windward side of the road. So, at full force there are constantly corrections.
5. They communicate in all the mayham. You see them talking through the radio and you can bet there is a lot of screaming and shouting.
6. Dealing with frustration. At full force, things always go wrong in all the mayham. The other one does not do what you tell him to do. There are deviations from the race line. It’s not going fast enough. X is not included. Etcetera, etcetera. That creates frustration. The natural reaction is what is mentioned in psychology: flight, flight or freeze. You get angry, you shut down (mentally), you cramp up, do not listen well, etc. Not giving in to these emotions, but staying functional, correcting mistakes of yourself and others, that’s the great art of encheloning at full force. Compare the World Championship Team Time Trials in Valkenburg (“I yelled, but he didn’t listen.” So what? Then you yell your louder, pull him by his shirt, whatever is needed.)
Knowing how it should be done is one thing. Doing it at submaximal levels is a skill that is quite easy to practice. Performing on full force and in all the mayham (like taking a penalty at soccer) is mentally much more difficult and requires much more attention and practice.
For the technique of enchelon and pinning the competitors on the side of the road, see a beautiful animation of a stage from the Tour de France: https://www.facebook.com/Fietssport/videos/1805709502974180/
About the psychological aspect and group dynamics, see also: psychology of cycling
About communication see also: http://www.smartercycling.cc/echelons/communicatie-en-samenwerking/