A pinned based column design doesn’t transfer any moment, or bending, load into the supporting foundations. It is carried by the steel structural and transferred into the foundation as vertical and horizontal loads.
Designing the steel building to a fixed base connection makes each column of a building like a flag pole; the entire moment load has to be carried by the supporting foundations. While this design makes for a cheaper steel building, the supporting foundation is required to be much more substantial and more costly. For a typical steel building, the overall cost of a pinned based design is more economical than fixed bases. Only under very specific, specialized building shape, use and location is a fixed base design more advantageous to a pinned base.