What's the point of a candidate key?

A key is called a candidate key, because while it could be used as a PK, it is not necessarily the PK.
There can be more than one candidate key for a given row, e.g., EmployeeID and SSN.
Often, rather than using a candidate key as the PK, a surrogate key is created instead. This is because decisions around what candidate key to use can be found to be erroneous later, which can cause a huge headache (literally).
Another reason is that a surrogate key can be created using an efficient data type for indexing purposes, which the candidate keys may not have (e.g., a UserImage).
A third reason is that many ORMs work only with a single-column PK, so candidate keys composed of more than one column (composite keys) are ruled out in that case.
Something that many developers do not realize is that selecting a surrogate key over a natural key may be a compromise in terms of data integrity. You may be losing some constraints on your data by selecting a surrogate key, and often a trigger is required to simulate the constraint if a surrogate key is chosen.

http://stackoverflow.com/questions/3961455/whats-the-point-of-a-candidate-key