Basically, sometimes there commands take a long time. It's nice to have feedback that something is happening even if it's just effectively a blinking light.
Speaking from experience, it's also nice to have feedback that the blinking light actually means something is actively happening, and not just that a process has been started and hasn't finished (it may have gotten stuck or suspended).
The only thing worse than a throbber that keeps going until everything is loaded regardless if anything is actively happening is a fake loading screen that lies to you about what it's doing. The only time you should use a throbber without context is when there's only a single action being processed and it's expected to take less than 5~10 seconds.
