Depends on the car, some Ford's have lots of them, many cars use them for cylinder head bolts (K series for one). One of my cars (Kia) uses them to fix the seats in etc.
You might not use them much but when you do you'll glad you got them.
Make sure they are seated fully before trying to undo anything. Very easy to chew up the bolt head, or the socket if it is cheap type. Can get very difficult to undo if the head gets chewed up.