Same for me, not quite sure it helps me to focus as much as it simply relaxes me enough to allow me to focus better. I think it might have to do with (for me at least) the effects sort of tamping down the signals from my body and allowing me to be more brain-forward if that makes sense. Also helps me cut some of the distracting noise from anxiety.

While I enjoy a drink, booze gives me splitting headaches within 15-20 minutes of drinking anything more than a small (40-50ml) amount.