What started as a practice to clear hard to get at brush, bring in the goats. Goats have been, and still are, used in the east to eat their way through underbrush, invasive plants, and poison ivy type plants.
In the west they are increasingly being used to clear hard to reach hillsides and areas around buildings to prevent fires from advancing. The goats to this in two main ways.
First, they clear almost all the underbrush, ground plants and the lower branches of trees. All of these things, particular invasive species, contribute to fires that have burned out of control. Remember native chaparral has evolved to burn hot and quickly. This allows for the germination of lower plants, and the fire moves so quickly mature plants and trees can survive a quick moving fire.
Second, they are voracious. A "trip" of goats can clear an acre of land a day. The homes at the cliffside in the video are from Anaheim Hills, but they could just as easily be from Malibu, Thousand Oaks or the Mulholland Pass. And they clear the hills as they can traverse areas that are inhospitable or impossible for humans to clear. And they are environmentally beneficial.
Anyway it is a cute video and goats are always cool.