Reasons not to use an auto-incrementing number for a primary key

Question

I\'m currently working on someone else\'s database where the primary keys are generated via a lookup table which contains a list of table names and the last primary key used

Answer 1

Using a unique identifiers would allow you to merge data from two different databases.

Maybe you have an application that collects data in multiple database and then "syncs" with a master database at various times in the day. You wouldn't have to worry about primary key collisions in this scenario.

Or, possibly, you might want to know what a record's ID will be before you actually create it.