They came when women were first admitted to the armed forces at all, and are contemporaneous with the rules barring them from combat roles. They are not the product of pressure to include them in combat roles.
Roles (including, but not, AFAIK, limited to, combat roles) within the military can have physical standards above the minimum standard for the branch of service, and no one that I know of that has argued for removing the restriction on women serving in combat roles has advocated against gender-blind role-specific standards.
The first is the one you're probably thinking of - you need to be so strong or so fast in order to do your job.
The second is more subtle, along the lines of "we want soldiers to have the discipline and willingness to follow rules that physical fitness signals." This is why they want a drone pilot, for example, to meet physical fitness standards.
Really, the military should have three fitness standards: one for all male soldiers, one for all female soldiers, and another for the soldier's specific speciality.
I've literally never heard the latter argument made by any proponent of women in combat roles. The closest I've seen is calls for the requirements for certain combat roles to be reviewed for appropriateness to the real requirements of the specific role; most proponents of relaxing blanket restrictions on women in combat roles I've heard will explicitly accept that, for most roles, any reasonable set of standards will still make it easier for men.