The superintendent is the GC's boots-on-the-ground leader — running the physical work, sequencing the trades, enforcing safety and quality, and keeping the field on schedule. Where the project manager owns the contracts and the money from the trailer, the super owns the build on the ground.
The PM and super are a partnership: the PM keeps procurement and paperwork ahead of the field, and the super turns that into installed work. Many people move between the two tracks, and field experience as (or alongside) a super is a proven path toward project management.