My friend puts cold rocks in his whiskey to cool it down. Is it to look fancy?
Can he just put the bottle in the fridge to cool it down without needing to put rocks in it?
It is to cool it down without dilluting it - ice cubes would melt. And yes, you could simply cool the entire bottle, but it wouldn't look that fancy.
Note that some purists would wrinkle their noses and insist that whisky1 is best enjoyed at room temperature and perhaps with a small dash of spring water.
1 And I'm soooo not going into a whisky vs. whiskey debate here.
The usual usage involves storing them in the freezer (much colder than the refrigerator) and using them instead of ice to cool your drink. When you use ice, you water down the liquor, which some people do not like. If you want the drink cold but not watered down, something like a whiskey stone is a good option.
Another thing is that they apparently cool the whiskey but not to the same degree that ice would, so you can taste more of the range of flavors. In fact, some people don't think they do a sufficient job of cooling at all.
As to storing the whiskey in the fridge, some people like their whiskey warm, so storing it in the fridge isn't necessarily a solution - it's much easier and faster to cool booze than to warm it up.