Question

what is the purpose of sipping coffee from a saucer instead of sipping from a glass or cup

Answer 1

you're supposed to put your coffee in a cup and set it on the saucer so you one, have a coaster, two so if it spills it goes on the saucer and not the table or yourself.

Answer 2

Because in a saucer, the surface is broad comparing to cup and here rate of evaporation is greater. Hence we are able to sip hot tea or milk faster from a saucer rather than a cup.