It's not about being rude, though I think there may be two seperate situations under discussion.
The first is: a lane is blocked a little ways ahead or a lane is disappearing. Some people (for reasons known only to God and the little green men speaking to them from their glove compartments) form a "line" a good distance back from the obstruction where traffic essentially stops. There's still plenty of good road surface in the blocked travel lane.
Not to be too blunt about it, but this is STUPID and does nothing but slow the flow of traffic even more than it needs to be.