This is a great way to upgrade your RAID array or move it/copy it to a new set of hard drives.
Eg. you have a current RAID 1 array on older/slower drives.
Just add at least 1 of the new drives to the array, update grub/install it and then boot into it. Then you have a transparent data migration that is fully synchronized.
mdadm --grow /dev/md126 --raid-devices 3
md127 : active raid1 sdc1 sdb1 sda1
153599936 blocks [3/2] [UU_]
[>....................] recovery = 0.8% (1281216/153599936) finish=119.7min speed=21202K/sec