A quite simple but reasonable example might be tropic, Mediterranean or at least in any aspects hot climate. Consider amazons, for example: in such a culture, there's no problem with a lot of exposed skin.
In general, in these settings, it needs for comfort.
Also consider agility: certain clothings decrease the speed of moving legs. Of course, certain military units are not heavily relying on it, but if you want to create agile and quick units (ninja/assassin/scout archetypes are the most obvious IMHO), then you may gain advantage from it.
I mean, whatever you wear below the waist, that might be beneficial is it's tight, because it's harder to tear down, and might provide effective protection, compared to some loose dress. However, it also slows you down because it can constrict you, by obvious reason. Try running with huge steps in jeans.
These were my main considerations while exploring the topic; of course, society is also a defining factor in it. Certain cultures (such as the "general European culture and mentality", if I can define such) find exposed thighs and buttocks (especially buttocks) suggestive and strongly attractive. Okay, some may argue about it but at the very least, one's for sure in general: in said cultures, people are always paying attention to it, rather than finding it "natural".
If this pressure is present, then exposed bottom won't (or less likely will) be present, especially among war-related workers (=fighters/warrior/soldiers/etc)
This is why I started my answer with hot climate. Exposed skin is less of a "taboo" in these cultures. In ancient Egypt, for instance, women often left their breasts exposed - and it's still not uncommon in jungle tribal civilizations. It's, on the other hand, less likely e.g. in Europe.
TL;DR - to justify exposed bottom, three main considerations can be taken: climate, agility and society.
Also notice that two of these make it possible, if not naturally expected for men to have less clothing below the waist. Of course, if males and females are equal in this question, then the latter has no significant advantage from it.
In the end, however, it's kind of an instinct in males to be pleased by the view of female buttocks.
I'd suggest trying to explore this very simple fact, if you want significant advantages for females.
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Have you seen this: new model army? - caution, may be NSFW.
– Mołot – 2016-09-07T20:55:17.5605
@Mołot this is better http://oglaf.com/matter/ do not mess with the fantasy
– MolbOrg – 2016-09-07T23:34:29.5534
If the question had been about exposing the abdomen, Stjepan Šejić has provided an answer (not a serious answer but an answer).
– Paul A. Clayton – 2016-09-08T17:22:44.6208Obligatory OOTS on "women's leather armor" – Mason Wheeler – 2016-09-08T20:09:59.043
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From what I see walking around at lunch time, I get a strong impression that a lot of women enjoy displaying their bodies. There's a lot of historical precedent for military uniforms optimized for display; in fact, before the mid 19th century, eye-catching display was the norm with military uniforms. A Napoleonic Hussar would have a different idea of an impressive military display than a future girl warrior, but both want to look good.
– Ed Plunkett – 2016-09-09T13:56:45.9272it's not a combat advantage, so not really an answer, but taking an unnecessary vulnerability can be a symbol of power. IIRC there are male birds that will sit in particularly visible and vulnerable spots, which is a declaration of superiority to other males that are not brave or capable enough to do the same. – Dave Cousineau – 2016-09-11T06:49:32.000
1An interesting addition is that ISIS believes it is 'dishonorable' to be killed by a woman. So perhaps the enemies could find it dishonorable to be killed by women and the fact they're showing their women makes it even more dishonorable. – Hyden – 2016-09-11T16:25:32.930
1I think it's quite obvious that the only reason such female armor exists - at least in games - is to entice male gamers. Thus your question embraces a certain naivete, albeit purposely. – LiquidMetal – 2016-09-15T03:43:50.983
This will generate too much hate to put in an actual answer: It helps with troop discipline. – ShadoCat – 2018-01-26T19:54:03.713