Both are tools used by Admissions to support class-building. They can send as many or few as they wish. A number of threads here recently has indicated LOEs are being sent out.
LOEs do exactly what they say - USMA encourages you to finish the rest of your application.
LOAs usually assure you if you complete specific things, such as obtain a nom, pass DoDMERB, pass CFA, etc., and are otherwise fully qualified, you will be offered an appointment.
The majority of cadets and midshipmen at the service academies do not receive LOAs. We like to think of them here like unicorns. If one wanders into your portal, rejoice, but don’t spend any energy trying to figure out how to get one. Put your effort into submitting your best-quality application as soon as practicable.
Do not look left or right and wonder why someone has received an LOA, and you know their (pick a stat) is lower than yours. Think unicorn free will.