Pole Buildings are just an alternate method of construction. Traditional buildings use a concrete or brick foundation, with stick-frame walls attached to it. The walls bear all the load of the roof etc, so doorways and windows require additional supports.
A pole building uses posts or poles set into the ground (often using concrete footings) as the foundation and as the primary support for the roof. Because the poles provide the structural support, it's easy to include fairly large openings without additional support. In extreme cases, they may be nothing more than a roof suspended on posts, with no walls whatsoever. Hay barns are frequently built this way. However, there's nothing preventing the builder from adding walls, doors, windows etc. and building a fully-enclosed building. It's also possible to pour concrete floors etc. and soon the building is nearly indistinguishable from a traditional building.
Because they don't require concrete foundations, which typically involve excavation, forms and various specialists, pole buildings are generally cheaper to build that other buildings, and are frequently encountered in agriculture, farms and other areas where large, inexpensive shelters are desired. Some states or cities prevent or restrict their use as residential buildings, because the pole foundation is not as durable.
If anyone's interested, I've built two pole buildings on the farm -- one is largely complete and enclosed, and the other is under construction. I could take pictures of them if it would help.
