Look at that... I'm not even finished with the reference sheet for Joyceste's new design and I'm already sexually harassing him like this xDxD

Joyce has to stay in shape at least rudimentarily for his job as a pilot of huge mechs, obviously physical prowess is not one of the primary requirements, but the stress on the body can be quite high if you're in one of those mechs for a longer time. Besides, the institution that runs all this is quite militaristic in nature, so I guess it's simply a standard requirement for all soldiers.