RAID5 vs RAID4 purpose of floating parity

There is a performance difference in that with RAID 4 each change requires writing to the single parity disk, which means things can queue waiting to update the parity data on that disk.

With RAID 5 you have a significant reduction in this because the parity update load is spread across multiple disks, so there's less chance if getting stuck in a queue.

Here's a nice link from Fujitsu with a short explanation and some nice animations to help clarify the performance/penalties of RAID 4 (as well as other RAID levels).