Before revision: It most frequently happens in order to qualify for a lower weight class ( usually in combat sports or rowing, where weight is a significant advantage) or in sports where it is advantageous to weigh as little as possible (most notably equestrian sports).
After revision: Or to meet the maximum weight limit in the heaviest weight class if one exists) (It most frequently happens in order to qualify for a lower weight class ( usually in combat sports or rowing, where weight is a significant advantage) or in sports where it is advantageous to weigh as little as possible (most notably equestrian sports).