There are many different types of metal sheds on the market. Some feature wooden framing, some feature metal framing, and some seem to have very little framing at all. While metal does offer a good degree of structural support to your building, it is nonetheless quite necessary to have a solid framing underneath the metal.
2×4 purlin construction is the method employed by our Amish builders. As you can see from this photo the overall framing system combined with the metal siding creates a strong building.
Notice in this picture from Domino Outdoor Living they are comparing two types of outdoor metal sheds. One has next to no framing and the other has two very thin metal purlins. While there are certain parts of the country where that type of construction may work well, neither are a good choice for Wisconsin or Minnesota.
Outdoor Shed Framing Requirements For Vinyl Sheds