My understanding is that it scans websites to determine what kind of technologies are being used (which hosting service, email providers, javascript libraries, analytics, etc)

This can be useful for market research, to see what technologies are being used and by whom. It could also be used as a lead generation tool.

Example: If I am creating a new email list tool, I could find websites (via builtwith) that are using a competitor and send cold email with specific comparisons between my new service and their current one, or asking what they like/dislike about their current tool.