GSTR-2B is an auto-generated, read-only statement that tells a buyer how much input tax credit is available for a tax period, compiled from the GSTR-1 (and other) filings of their suppliers. Because it is static for a period, it gives a stable basis for claiming ITC.
Reconciling your purchase records against GSTR-2B — matching what you recorded to what suppliers actually reported — is how you avoid claiming credit you are not entitled to, and chase suppliers who have not filed.