Pole Barns are common structures that use posts and beams to support the roof structure. The cost savings come from fewer or no studs and foundational support in piers verses footers.  Both of these aspects require fewer materials but also require less labor  in construction.  Originally the posts were tree trunks or thick straight branches.  Now, they are typically pressure treated 6x6 or 4x4 lumber posts.  To provide support for siding, pole barns typically have girts.  Girts on walls and purlins on trusses and rafters provide a base for affixing siding and roofing.