What do you think the boxing match was for? Vader is testing Darth Back. If Darth Back can defeat Vader then he proves he is strong enough to be his apprentice; if Vader can beat him, then is useless and deserves a pummeling.
Lucas re-used the same idea for Episode III. Palpatine is testing Anakin with Dooku to see if he is strong enough but Dooku doesn't know it.
"It's like poetry, it rhyms."