Generally speaking, there are three main entities involved in the immigration process:
- USCIS: they verify the petitioner documents and relationship to ensure that the petition is valid.
- NVC: they verify the applicant (intending immigrant). Their job is to guarantee all documents needed are there for the interview. That's why they request the affidavit of support forms, police reports and others.
- Consulate: they have access to all documents and have the final decision to either accept or deny a visa once the interview is done.
So as you see, each of these have their own role. Since there are so many visa types and documents for every country, it's necessary to split the process into multiple steps.