newbie rockwool question

I've noticed a lot of posts and how-to's of rockwool cloning and seeding. Most of them shows the rockwool with an outer plastic layer, typically white. Is there a reason to this? The rockwool I bought doesn't have any outer layer, is there any concern? If there should be an outer cover, what should I use to fix this?

Some blocks are wrapped.. No worries..

the wrap slows down the blocks drying out, i remove it once the clone has rooted enough to pot on into whatever, i use lecca in mesh pots, or more rockwool, or even soil.