Self-timing is great for static shots (if you have the time to wait for the camera to fire) but many shots have moving objects, such as clouds, waves and people.
If you want to capture such images at a particular moment, such as, when the light comes through a gap in the clouds to light up a feature in the landscape, self-timing, even for 2 seconds, may mean you miss the moment.
With a cable release, you could take two shots in even just a 2-second delay.
Horses for courses!