Rebalancing of managed disk groups has to be done whenever you add new mdisks to a group - otherwise the IOPS and response times on these managed disk groups become unbalanced and you will not see the expected performance out of the group.
More informations about unbalanced mdisk groups here
in this picture you can see that most of the IO is handeled by only one managed disk. It handles three times more IO than the other mdisks. This mdg will become a performance problem soon when this one mdisk becomes more load and saturates.
This is how it is lookiong like in performance view. The green lines are IOPS from the one mdisk and another with smaller load and thge red lines are the response times. It is obvious that response time of the high load mdisk will become an issue soon.