In principle, overbooking does not happen in Ryanair and most LCCs.
There might be other reasons but one of them is to assure that you take your hand luggage with you. Ryanair charges extra for checked-in luggage therefore almost everyone carries hand luggage. This used to be a problem because there is not enough space inside the cabin for everyone to put their luggage. Currently, hand-luggage that does not fit goes to the cargo hold. They usually start distributing the tags to put the luggage on the cargo hold among the last persons in the queue.
If you want to be sure your luggage flyes with you the only way is to queue early (or buy the priority boarding).
If your luggage goes into the cargo hold under these conditions you don't need to pay extra. You deliver the luggage when you walk to the airplane.
NOTE: currently Ryanair charges to take hand-luggage (that doesn't fit under the chair) inside the cabin. Only a limited amount of hand luggage is allowed. The process became predictable and you will allways take the luggage inside if you paid.