iirc having high hp and larger than normal size (3) they get targetted by the target monsters command. (If you have no larger/more hp units). This can prevent them from targetting your commanders if these have above average size and hp. (If you are playing Lanka for example, and don't use the normal size 3 apes units that much). (They also have access to dark vines which are better, but lets disregard that
).
So basically, they are shields/decoys. They lack the protection to be real tanks. (but there are spells that help them with that of course).