Standard reports (profit and loss, balance sheet, income statement, and the rest) update live off your books and let you compare periods, filter by tracking category, and export to PDF or Excel. For most monthly close routines, this is genuinely enough.
The custom report builder goes further: you can start from a template or from scratch, choose your own rows, columns, and groupings, and save the result as a reusable management report. Xero also supports report templates, which matter if you manage several organizations (a bookkeeping firm with multiple clients, say) and want the same report structure applied consistently across all of them rather than rebuilt one org at a time.
Where it stops: cross-entity comparisons for businesses running more than one Xero organization, anything that needs non-Xero data sitting next to the financials (sales pipeline, headcount, operational KPIs), and highly specific board-level formatting that the builder's layout options genuinely can't produce no matter how much you fight with it.