There are a lot of companies that use WSS instead of the full MOSS (Std or Ent) for a variety of reasons. There is a big difference in features, but not all features are needed in all situations.
From my perspective I typically think about WSS working fine with very small companies, work groups or organizations that want lots of loosely connected sites.
Having MOSS adds a layer to help manage the system and content across those sites. The services in the Shared Service Provider for example offers shared User Profiles that can be managed centrally and shared across all site collections. Search is much more flexible, and then there are the other advanced Enterprise features.