Sequential scrub turns "let's read all the blocks in the order we see them in the metadata" into "let's separate this into a read and a dispatch phase, so we can group things we dispatch into sequential regions and make spinning disks less sad at the amount of random IO needed".
It's quite a dramatic improvement for pools on hard drives.
Very much so. Spinning disks are bad at seeking randomly, and turning lots of random IOs into a smaller number of relatively sequential IOs is a substantial win.
