Different perspectives. New ideas. A larger pool of creativity. Not everything people have to provide is merely physical. The more you limit the pool of people eligible for a career, the less brain diversity you get. That leads to a more limited pool of talent and ideas which can ultimately impact war-fighting capability as some good ideas may never reach the pool.
There are so many variables beyond just push-ups and pull-ups to define a good warfighter! Why don't we limit a war-fighter to >220 lbs, >6'5", able to bench 300lbs cause they have the greatest skull bashing and brute force power like some old civilizations did? Because there is more than just being big and strong. I even look at myself. I maxed the pull-up test at USAFA for several years but have quite a bit of trouble tossing someone my size off of me. While the pull-ups make it look like I am very strong, they don't realistically represent my actual strength, at least in throwing a person off of me in wrestling.
Limiting the talent pool can limit war-fighting capability too. Again, why is it imperative to have a female personnel officer or a female intel analyst have to meet the same standard as the male? If she is fit by female standards, her inability to lift an additional 20lbs over the male, if she is not in combat, is moot. If you keep her out because, despite being fit, she can't bench 200lbs, you may have just lost a very intelligent and effective part of the war-fighting whole.
I don't like to view war-fighting abilities so narrowly. That's why I think we need empirical evidence and to evaluate if the assumptions are really TRUE.