Reading the page, it looks like your document gets put into a queue for processing, and then you can download it after. I imagine they wanted to avoid all the hassles of emails, as well as having accounts to prime the system if they decide to enhance it as a for-pay service.

It probably also stops assholes from using the service on PDFlower's dime for their own needs.

There are plenty of services that put my job in a queue and simply update the page via JavaScript to show progress and deliver the end result. The better implementations identify me via session cookie and recognize me when I check back half an hour later to get my results. That's a much nicer experience than creating another account at some random service that I might never visit again.