The 10-Q is reviewed rather than audited, and its notes are abbreviated. It is still the only place where the quarter's balance-sheet and cash-flow-statement appear in full, since press releases often show only the income statement.
Filing usually lands days or weeks after the earnings-report, which means the market has already reacted to the headline numbers while the details that change a thesis are still unread.
Example: Northwind Tools reports in late January and files its 10-Q nine days later, disclosing the inventory split that shows finished goods up 26% while the press release showed only the total.
Related: form-10-k, earnings-report, footnotes, edgar, form-8-k