What is ROWGUIDCOL purpose here?
I tried googeling it and couldn't find anything that explains what ROWGUIDCOL does.
It's a special marker which tells to a merge replication that this column should be used as a synchronisation key between publishers' and subscribers' versions of data.
Neither the column itself, nor the data in it are affected by this marker in any way. Think of it as a bit flag being set somewhere in the column metadata.