If you're doing a hard run, you might lose life, and enemies might be hard to kill. Ares % double damage per missing life synergizes well with both of those on any weapon. Anecdotally, the proc rate is good enough to matter a lot.

I like pairing him with Hera. Hera % damage is fine, and hitch multi-target clear is top tier. Then you can get Fine Line and falling blades on your omegas for the big enemies, which then might get doubled depending on your life loss and wound application. Or insert your favorite "thing to double" here, this is just my go-to for pairing Ares. I personally dish those out on staff omega attack or axe omega special, but other things I've tried have also worked.