well, you need muscles to move right?
I'd say the role of strength is to be the means by which you can correctly position yourself so that you do not use "isolated" muscular force to unbalance or "take down" someone
upper body strength pits your pectorals, biceps, triceps, deltoids, agains someone else who <if attacking correctly> is using their entire body.
the preferred strength is that of the entire body aligned with gravity .. which is pretty powerful compared to say biceps alone.
so easy to talk about, so much work to achieve..
<little less newbie alert>