Usually the MOC will only open up one, maybe two slots to fill for each year. A slot, once filled, doesn't open up again until that cadet/midshipman leaves the academy, and MOCs need to make sure that there is always at least one slot available each year.
Depending on the nomination method your MOCs use, the type of nomination may also be important. A principal nominee is guaranteed an appointment if otherwise qualified (medically, scholastically, physically). If the principal nominee is given an appointment, the other nine will go into the national pool. If the principal nominee is not otherwise qualified, USNA will either go down the list in order (in the case of numbered alternates) or choose one of the other nine based on WCS (in the case of competing alternates).
If the nomination method is the competitive method, the best nominee (that is otherwise qualified) will be appointed based on WCS.
Even if you are not the principal nominee, if you are one of the other nine, you are still considered a candidate with a nomination. Thus if you have an LOA, that nomination, along with any others you may have, will fulfill the requirement for a nomination and lead to an appointment. The more nominations you have, the easier it is to match an LOA with a nomination and the easier it is for USNA to award your guaranteed appointment.