Granny style

I recall my grandma drinking coffee this way. And this was a common style in old times when cups were smaller and saucers used beneath them. Nowadays people use large mugs with no saucers. And if using sugar, it is stirred into the coffee before drinking it.

The secret of this style was for three reasons:

Cooling - When poured onto the saucer, the coffee got cooled down.

Aerating - It was important to "slurp" the coffee noisily, to cool it some more, but also to suck air to make the coffee tastier.

Sweetening - When holding a lump of hard sugar between your teeth, the sweetness spreads into the mouth differently than if mixed into the drink in advance. One has to test this to believe it. Although, personally I prefer not to use sugar at all.

Sources

➽ Why you should slurp coffee
➽ How to Taste Coffee Like An Expert
➽ Drinking Coffee From A Saucer is an Old Tradition Your Grandparents Probably Did
➽ [Swedish] - "på bit" - drinking coffee with a lump of sugar in your mouth or between your teeth
➽ Jaakko Kolmonen ja kahvin ryystäminen (in Finnish)