A single scan often contains several unrelated documents. Splitting them apart makes each one sendable on its own, and much smaller.
People often scan a whole folder in one pass: a passport, two payslips and a utility bill in a single sitting. That is efficient at the scanner and awkward everywhere afterwards, because you can rarely send the whole bundle to anyone.
Splitting also solves size problems. A form that rejects a twelve megabyte bundle will happily take the three megabyte section it actually asked for, and you have sent less of your personal information in the process.
Send only the pages that were asked for. It is the simplest privacy measure there is, and splitting is what makes it practical.
Yes, select a single page at a time and save each one.
Yes, the pages are copied rather than re-encoded.
No.