Drinking, like smoking, can be an 'associated habit, all this means is that we have learned to link the two with a certain task or function.
I know many players that do not drink, but put their instrument in their hand and the feeling for the need to have a drink appears. It is like a ritual they have associated with what they do, it may be a drink before they go on to play, nit equaly could be go for a ciggarette, or go to the bathroom, wash their hands etc...but they feel un-comfortable if they cannot do it.
If it quietens the mind, then go for it, having a drink to calm the nerves may just be a ritual that calms anxiety, maybe that anxiety occurs sometimes because there is no option to have a drink, so expectation, or the lack of it, plays a part.
Superstition????? Well maybe.....Pre-gig routine???....Defiantly if it becomes part of what you do.
The same can apply to giving up something as it does to adding something that we believe helps us.
So if giving up drink for the night and the gig goes well, reinforces the idea that it was a good thing and to continue with it.
Equally if we have a bad experiece it reinforces that we cannot give it up.
Playing the same set night after night is the same skill set, but the situation within which it is applied is different. So having a regular routine is familier, comfortable.....it settles us down and tells us "we have been here before, done this before, all is well and no reason to believe this will not be any different".
In the end it is the power, or the importance that we put on such things as to whether we believe any change will make a difference, we can of course say and do all we want, but id deep down inside we do not believe in what we do, it will surface a some point when under real pressure and expose us.
Since we are there to play music and entertain then that should be the focus, if we takes care of that then everything should fall into place....but that is not life, so we adapt and change to suit circumstance.
If that means sometimes we have a drink, then so be it, if it means sometimes we do not...then so be it...but it should never be an absolute of one or the other....that will bring a different set of issues, and if those issues affect the music or the playing then the original point is lost and giving up drinking has not stopped the issue, just changed it for another issue.
Each to their own, but just be aware of what is going on around you and act accordingly to suit the situation in your own thinking.