It is supposed to provide increased security for users that don't know what they are doing. But because it's not feasible to have humans review every single change made to every single add-on in a reasonable time they've automated the system. So now instead of providing security all it does is make a walled garden where everything has to be approved by mozilla but no one actually checks what they're approving.

So it both provides a false sense of security and prevents users from being able to control the software they run on their own machines.