Here is my understanding about why INS wants I-140
It is actually not the approval (I-797 ) itself but the I-140 immigration petition form when your lawyer or your employer sent to INS on your behalf, I think INS wants it is because it has all detail information regarding your job, such as salary, job description,
job location, company info , etc, when IIO adjudicate your 485 , he/she is supposed to look at the original petition , then match it with your labor certification, and your latest employment letter before your 485 can be approved, 140 approval alone doesn\'t give so much details , it just says whether you\'re EB1 or EB2, EB3.
Once you choose CP on your I-140 petition form, approval along with original petition will then be forwarded to NVC , NVC will forward it to Consulate. In this case, INS most likely will not have the copy of your 140 petition but just a record of your 140 approval , but if you choose AOS on I-140 petition, then INS has everything on file.
That\'s why we see AOS I-140 people got 485 approved faster than those CP 140 cases.
This is just my 2 cents, but I got this proved actually through some of my friend\'s experience. Any other thought ?