what the following update means? thanks

several days ago, I got online update that the card was ordered. But today the case was updated again as post-decision activity which is not the final step(card ordered). The update is :


Post-Decision Activity

On April 6, 2010, we mailed you a notice that we have approved this I485 APPLICATION TO REGISTER PERMANENT RESIDENCE OR TO ADJUST STATUS. Please follow any instructions on the notice. If you move before you receive the notice, call customer service at 1-800-375-5283.

For approved applications/petitions, post-decision activity may include USCIS sending notification of the approved application/petition to the National Visa Center or the Department of State. For denied applications/petitions, post-decision activity may include the processing of an appeal and/or motions to reopen or reconsider and revocations.


I do not understand why the USCIS sends notification of the approved application to the National visa center?

Thanks