Purpose of IDs in a User Table Database
I'd answer with a question: Why do you need a table for a single row?
Regarding the design of the table, any table should have a primary key. I won't discuss that as it's the baseline of any design book. Regarding the way of designing that primary key, there are two schools of thought:


one that makes the PK as a separate column, usually self generated, like a GUID or and auto increment INT (in your case a separate uniqueidentifier column);
one that makes the PK as a column (or set of columns) internal to the table (in your case would be a username or email or SSN, whatever makes that user unique) that uniquely identify a record.


To what line you adhere, it's only a matter of taste.