You quoted it once, then keyed it again as an order, again as a delivery, again as an invoice
Nobody decided to run the sell side across four systems; it accreted. The quote is built in one place, re-entered as a sales order in another, the warehouse works from a third, and finance raises the invoice from a fourth. Each hand-off is a retype, and each retype is a chance for what you shipped to stop matching what you billed. It holds together until someone asks the one question the four systems cannot answer between them, what did we deliver this quarter that never got invoiced?, and the answer is a reconciliation nobody scheduled.
What changes with order to cash
The quotation and the sales order are the same record. What you set up once, the lines, the options, the validity, the signature, the deposit, is what the customer signs and pays against, and confirming it changes a state rather than re-keying a document.
What shipped, what is billable, and what was delivered but never charged all read off that one order, because the delivery and the invoice are linked to it rather than copied from it.
You stop chasing overdue invoices from a spreadsheet kept beside the ledger. The invoice comes off the order, and the follow-up on an overdue one is drafted for you against the ledger itself, drafted, then held for a person to send.