Parentheticals (also called “wrylies” or “actor’s direction”) are used to clear up confusion about a line that could be read multiple ways:

They can also be used to indicate who a speaker is addressing:

You will sometimes see them used to indicate action. This is acceptable if the action is short. (For longer actions, you’re better off ending the dialogue block and putting the action in scene description.)

This…

…can be replaced with:

Parentheticals are also sometimes used to indicate a pause in dialogue:

Or with more specific action…

The same idea could be achieved by using a line of scene description:

Parentheticals should be used sparingly.

In this 2010 blog post, John August takes a look at some professional screenplays, and counts their parentheticals use.