They weren't used to get a straight edge at all LO. They were used to make the staves for large barrels and kegs. You pulled the timber along them to taper each end.
Most of the shipwright ones I've seen weren't that large, probably because the timber was too bl---y heavy to push!
Some large wooden planes had dowels sticking out on both sides, two ropes and an apprentice were attached for pulling!
Roy.