MCTS 70-433 IDENTITY Columns

I strongly suspect that it is SQL Servers dependencies on Index Oriented Tables (aka Clustered Indexes).

Anyway Identity columns are an easy way to create employee or customer numbers automatically.

Occasionally, for example you are loading/migrating existing customers or employees from another system and you don't want to use the automatic identity to allocate a new customer or employee number:

SET IDENTITY_INSERT ON;