The outward side
The GSTR-1 summary comes out grouped as the return needs it — B2B invoices with each buyer's GSTIN separated from B2C, credit and debit notes listed, an HSN summary built from the item records, and tax broken down by rate. Because the rate sits on the item, a month of mixed-slab material bills summarises correctly without anyone classifying anything by hand.
The inward side
GSTR-3B needs outward tax and input credit together, which is where shops that only record sales get stuck. Purchases entered properly capture input GST on every supplier invoice, so both halves exist in one place. It also lets you see, before filing, whether your input credit looks like what you expected — a large gap usually means purchase bills that were never entered, and finding that before the return is filed is considerably better than after.
Reconciling against 2B
Input credit is only available for invoices your supplier actually reported. With every purchase recorded against a named supplier with its invoice number and date, comparing your records against the portal is matching two lists rather than reconstructing one. The differences are usually a bill you never received or a supplier who has not filed, and both are worth a call while the quarter is open.
Keeping it clean the rest of the year
Most filing pain is created eleven months earlier: bills raised outside the software, HSN codes left blank, cash sales with no party, purchase invoices in a pile. None of them hurt on the day and all of them surface at return time. The advantage of doing everything in one system is not the report at the end — it is that there is nothing left to reconstruct.
Before you file, three checks
Compare total sales in the summary against your own sales report for the month; a gap means bills raised outside the software. Check that input credit is roughly what you expected; a shortfall almost always means purchase bills sitting unentered on the desk. And check that no B2B invoice is missing a buyer GSTIN, because those get rejected or land in the wrong bucket. Five minutes, and it catches the errors that would otherwise be found by a notice rather than by you.