On many country roads there is no space to pass in the same lane if riders are several abreast but plenty of room to pass (not squeeze by) if they're riding single-file. And going fully into the opposite lane could mean trouble for whatever reason (poor sightlines, somewhat heavy traffic in the other direction...) If you can help make passing faster and safer, why not do it?

However, if there is another lane going in the same direction and there are no laws about riding two or more abreast - might as well stay in a formation.