Drees,

You can create an user without an employee,  its not mandatory to create an employee for every user(as they might not be), some user can be customers, some can be suppliers too.

But if you are creating the user for own company then you will need it to be associated with an employee too,

I am mentioning some reasons for that, which needs it to be an employee, there might be many more..:

1) To assign a user to a department in a company(employee master).

2) To get the timesheet approved (which can be done by user's manager, which is assigned at employee master)

3) Working Address, home address, performance, qualifications, and many other things related to that users, which give a depth information about his professional and personal life....

There must be more thing which are using users employee for functional implementation.

Hope it will help you in creating a user, with/without an employee!!