The most obvious advantage is the superior insulating quality. Not only in the R value but in the much under appreciated moisture barrier that becomes very noticeable when the walls are sheet metal.
It makes your entire building more structurally sound. Light hail that would otherwise dimple the metal may now bounce off. You will also notice less creaks and moans during wind.
The interior is hard and durable. If you so much as bump against fiberglass bats you can easily knock them down. On the other had you can run you lawn tractor into 2lbs foam and do little but leave a small dent.
Last and definitely not least is fiberglass bats just don't fit! Spray Foams ability to fit in any space imaginable makes it the ideal insulation for a pole barn. It is just next to impossible to fiber bat insulate a pole barn with the narrow wall construction that makes them so versatile.
