Invoices identify themselves
At sync time, invoice XML attachments and notices from the tax authority's portal are classified automatically. You do not search the inbox for what a supplier sent — what is an invoice appears as an invoice.
A thread links to what it is about
From any conversation you can create links to a contact, customer, invoice, project, deal, document, contract or court case. Correspondence becomes part of the relationship history rather than a parallel store only its author can search.
Privacy by default, not by option
Remote images are blocked by default, and when you load them they pass through a server-side proxy that hides the reader's IP address. The tracking pixel in a marketing email learns neither that you opened it nor where from.
Awkward formats — including Outlook's own packed attachments — are unpacked properly rather than shown as an unreadable file.
Scheduling and deliverability
Messages can be scheduled. Because sending uses the same infrastructure as invoices and campaigns, domain authentication and reputation are managed in one place.
Worth knowing
An email client separate from your accounting makes you the bridge: find the invoice in the inbox, download it, upload it somewhere else. Here the bridge does not exist, because the second system does not exist.