Difference between a user @ child BU and @ root BU having System Administrator role

First thing why do we have System Administrator role available at all the child Busiess Units?

Secondly, I don't see a difference between aforementioned two users e.g. user @ child BU with System Administrator role can access all the records across all BUs just like System Administrator at rool BU.

Is my understanding correct and is it expected behaviour?

Thanks