Can someone explain the use/importance of the “probationary period” for Fed Civil Servants?

So the administration has been firing many folks who are under probationary status. To a layman (like me), probationary status usually means length of time (3months to 1 year) which a new employee is being evaluated for fit and performance. This is generally speaking. It sometimes applies to new kids on the block (ie fresh college graduates) but could also apply to new employees to any organization regardless of experience. Which makes sense.

However I have learned from anecdotes from all the recent firings that 1. The Fed probation length can be as long as TWO YEARS. 2. Even if you change departments within the same organization, you go on probation (eg you move from 1 dept to the other in the FDA). This doesn’t seem to make sense to me. Is there a logical reason for this?

I mean I get it if you move from Wildlife conservation to FDA but if you move within the same organization, it shouldn’t be that long. In fact I’ll argue there should not be one.

I’ll confess i don’t have all the facts here but this doesn’t make sense to me. Or is it all about union protections/benefits?