You typically need it when you receive a single file that actually contains different documents: a scanner that produced twenty invoices in one PDF, a register with one form per page, a batch of receipts to be filed separately.
The tool creates one PDF per page, numbering them in order, and gives them back to you in a ZIP archive that opens on any computer without extra software. The pages keep their original size, orientation and quality.
If what you need is a single document with a few chosen pages, use Extract pages instead: split produces many files, extract produces one.