What is the purpose of performing static fire of Spacex star ships? ?

To safely impress the public and serve as promotional material for one man's financial aspirations. Static firings are successful and impressive. Spacex rocket firings usually blow up. Not good publicity there.

Engine test. It's better if you make sure all the parts work before you fire the big explosive missile into the air.

Basically, to check the engine works as expected. You don't want the first time you try it out to be on an actual launch.