Muscles, bones, tendons, cartilage, and ligaments are all made up from the same basic stuff but in different ratios. So tendons are like ligaments, the defining difference being ligaments connect bone to bone, tendons connect muscle to bone.
The main purpose of ligaments (besides stopping our mass from flying apart when we smash into a more solid mass) is to transfer force from one bone to another.
Since all our bones grow into a spiraling shape, the ligaments continue those spirals from bone to bone, transferring force through the continuity of a given spiral. It is key to the effective transfer of force that weight does not go into the joints. Thus if your opponent touches you or puts their weight on you--all of that force should travel in a spiral through your bones to the ground.
Whenever you transfer force from one bone to another, it is the ligaments that do that job.