Would having backwards hands (Like a Raksasha from D&D) provide any kind of tangible combat benefit.
I kinda doubt it. Both my editions of the Monster Manual insist that it doesn’t affect their manual dexterity, which, I’m not so sure about that. They suggest that all it does is make the Raksasha look more disturbing.
I mean, if you spend enough time, you may be able to come up with some extremely situational examples, where their reversed hands would be an advantage. For example: They can claw you on a backhand, instead of a normal rake. Though, the value of that is kinda dubious. Mostly this detail is just to make the Raksasha more memorable and feel more unique. I mean, the 3.5e MM runs to over 300 pages, the Raksasha needs something to stand apart from the crowd.
Details like this can help to sell a fantasy creature you’ve created. Weird little anomalies you can use to make your world feel less generic. At the same time, these don’t need to be immediate, concrete, beneficial powers. Things like the Rakshasa’s reversed hands can just be there to sell your setting.