What would make an empire invest time and effort to create this kind of force?
It wouldn't, because you've stacked the deck.
You've picked a characteristic to hang magic on which is specifically discriminatory against women. That doesn't make you a bad person. :) But it does mean the worldbuilding you come up with can never have women involved in front-line warfare as a matter of course. Women cannot compete on physical strength, and they cannot succeed through magic. In a swords-and-sorcery environment, this very much relegates women to the position of stuff being done to them, not having agency of their own.
Our world does have a few examples of women who could fight alongside men on equal terms. Caster Semenya famously has relatively high testosterone levels, for example. These women are extremely rare though, and whilst they outclass other women and most men, they still cannot compete at the same level as elite men. Tennis is a relatively good comparison here, because it requires significant strength as well as skill and endurance, and we do have some straight match-ups for comparison. Billie Jean King beat Bobby Riggs in a straight contest, but that was pitching the best woman in the world against a 55-year-old. Against any seeded male player, King would have been comfortably beaten, and she was perfectly well aware of this; the point was to take down the arrogant Riggs, not to prove she could be competitive against a top-ranked male player such as McEnroe or Borg. In 1998, Karsten Braasch (ranked 203) beat both the Williams sisters (then ranked 5 and 20) in straight sets, back to back.
Women can compete at the same level or even beat men in ultra-endurance events - but this is because they do not have quite the same blood chemistry and body composition as men, and the lack of testosterone is a positive advantage. We also have plenty of examples of women historically who were notable military commanders and strategists, but this is not the same as fighting on the front line.
Of course this all assumes that the empire invests the time and effort. Historically it was certainly common for rich people to have high-quality, highly-ornamented armour made, in spite of them not being particularly skilled fighters. You could certainly make a case for this being a fashion statement adopted by trend-setting aristocratic women, in the same way as women wear military-inspired outfits today. They would not actually be fighting, but they would have the money to buy the best equipment.
You could also make a case for aristocratic men setting up their own group of all-female bodyguards for their own reasons, as Muammar Gaddafi did. The common male fantasy of being surrounded by powerful women willing to die for him could well be a reason! It's not a fantasy of mine personally, I should add, but it's definitely a thing which some men like. Bodyguarding also is more about deterring attacks by regular citizens, and often spotting trouble before it starts, so raw power is less important than intelligence and situational awareness, and women are equally capable here.
What you can't make these women though is a practical front-line fighting force.