Most Helpful Girl
Getting nails done, or hair done, or any similar treatments isn't really to impress guys. If we're looking to impress anybody, it's our girlfriends, since they'll be the ones to actually notice. More predominantly however, as a few others here have said, it's just a nice feeling to be attended to for a few minutes and to come out knowing that you look good.
Personally, I'd totally go out and get my hair and nails done on a weekly basis if I had tons of expendable cash just for that latter reason.