Some states, like Texas, are extremely picky about who calls themselves an engineer, irrespective of industrial exemption (which the vast majority of people fall under). PE licensure in other engineering areas (like civil engineering) is required to offer services to the public, helps when offering expert testimony, and can be required for principals at consulting firms. The software engineering PE is an extension of IEEE's efforts in software engineering, and is probably a good thing for safety-critical industries.
Since passing the fundamentals of engineering (FE) exam is (usually) a prerequisite, someone who has a PE in software engineering probably knows their way around a free body diagram or Laplace transforms or MSD systems, i.e., stuff the other engineering fields do.
