Sheet Metal Fabrication vs. Using Channel

[FrankJ]

A couple of reasons for channel & I-beam for construction are the shapes can be rolled rather than extruded. Bolted & riveted connections are easier than with hollow shapes. I-beams are useful when most of the loading comes in one plane.

With structure, a lot of times I end up sizing for stiffness rather than strength making yield strength less important.

All of these shapes exist for the simple reason is that they all are useful for specific situations.