I have done the same with coffee or even wine - it is out of politeness I would say - when we are invited to a coffee or dinner with colleagues or friends, I feel I have to be polite and order something appropriate for the setting and sip or pretend to so I'm not the only one drinking water at the table - it can be awkward/ get some snarky comments (you've lived around here so I have a feeling you'll understand what I'm trying to say)