Presumably you had to travel to get there. It’s likely you didn’t have anything to drink while travelling. So they offer you a drink.

Also, it’s nice to sit and chat while enjoying a drink, whether that is a warm drink (coffee, tea, whatever), a cold drink (water, juice, soda, whatever), or an alcoholic drink (beer, wine, whatever).

Edit: Also why are you annoyed? If you don’t want anything a simple “nothing for me, thanks” is fine and shouldn’t put you out too much!