Your customer needs to pay you for a product or service you’ve delivered. You need an invoice.
You type a bunch of numbers into a structured form and it stores those numbers in a structured database (most likely your accounting system).
Then you turn that structured information into a PDF invoice. An unstructured piece of digital paper that gets emailed to your customer.
Your customer then opens that invoice, reads it, and manually enters those numbers back into a structured database (their accounting system).
It’s like taking the sack out of your kitchen bin and emptying it onto the street. The bin collection people arrive, pick up each piece or rubbish, put each one into a sack, and throw it into the truck. You could have just given them the sack.
And invoices are one of the easiest, best-managed types of business document. Heaven help the rest.