It might be quicker or more convenient, but many of the roles of a good bartender have nothing to do with taking money from customers.

“An efficient bartender’s first aim should be to please his customers, paying particular attention to meet the individual wishes of those whose tastes and desires he has already watched and ascertained; and, with those whose peculiarities he has had no opportunity of learning, he should politely inquire how they wish their beverages served, and use his best judgement in endeavouring to fulfil their desires to their entire satisfaction.”

– Jerry Thomas, 1862

http://www.telegraph.co.uk/business/2017/09/25/reading-university-introduces-self-serve16-tap-beer-wall/