If what I am thinking is correct, you tend to see it more overseas. And I believe it is done to help preserve the equipment.
With that being said, if a match or tournament is started with the use of a rack, I would think that it should stay that way for the duration of the match. Also I would think that you could change racks either because the outer dimensions of the rack may be slightly different and it may change wheather or not a breakball is inthe racking area.
Hope this helps !
-steve