I buy girls drinks but only when it's not the primary intention.
I don't do that whole thing where I don't even know a girl's name and I just approach her randomly in a club and ask if I can buy her a drink, at which point she ponders whether she's cheap enough to say yes purely for the drink alone and not actually to show interest in the guy.
I mean, a girl would be absolutely stupid to think a guy is offering her a drink for the sake of offering her a drink like it's some charitable cause. It's obvious when a guy asks to buy them a drink what they're really asking is if they're interested (for whatever, to make friends, to pull, ONS etc.).
By saying yes the girl's reciprocating that interest (or at least that's the impression a guy gets) but there are some who just take the guy for a mug because they're too spineless to pay for their own drinks so resort to just accepting.
I only buy drinks when I just happen to get chatting to a girl or we've spent a bit of time dancing without actually having formally introduced each other. This confirms her interest is genuine and I'll ask her if she wants to head over to the bar where we can have a chat and of course drink, I'll happily buy her drinks then...even if we're just being social to make friends or whatever doesn't necessarily have to lead to sex.