What’s the big deal about passing probation?

I know you become a full time permanent employee after passing the probationary period, but what does that mean exactly? I read that you can always go back to the position where you passed the probation but can someone give me an example? So if I pass my probation as AGPA and decide to go become HPS but don’t like it or fail probation, I can go back to AGPA, but does that mean that the agency MUST hire me or do I still have to wait for the agency to have an open position? There was Newsom’s budget cut and a vacancy sweep, so how does that work if I passed as an AGPA but decide to go elsewhere and decide to come back? And how easy is it to move to another agency with the same position if you passed the probation? Any other benefits for passing probation? TIA.