Companies provide another level of information about users. When you create a user you can assign the user to a specific company.
Companies are not compulsory in the knowledgebase. You would use them if your users were from a small number of companies, but if your users came from all around the world and were a mix of individuals and company users you probably would not bother.
Some examples where you might user them:
Some examples where you probably wouldn't:
Article ID: 81
Last updated: 19 Dec, 2012