Often, older electrical panels become full and all the spaces are filled with circuit breakers. When this is happens, you cannot add an additional circuit. If you wanted, for example, to add an above the oven microwave (which requires its very own circuit breaker) this wouldn't be possible AS-IS. There are many upgrades or items you might want to add which require an additional circuit, from their own circuit breaker.
Here are some more examples of needing an additional circuit:
There are many reasons why you want to be able to add a circuit, circuit breaker.
If your electrical panel has no spare space, you are really constricted as to what you can do.