EDGAR is the primary source behind almost every data provider. Full-text search covers filings from 2001 onward, and each company has a permanent identifier that makes historical work straightforward.
Its practical value is speed and fidelity: the filing appears on EDGAR before it appears anywhere else, and it appears exactly as submitted, without a summary writer between you and the disclosure.
Example: Northwind's 8-K announcing the CFO departure hits EDGAR at 4:31pm. The first newswire summary appears eleven minutes later, by which time the shares have already moved in extended trading.