What are Sign-up Requests? 
Signup Requests are records that are either a) not already in your RE database, or b) are in your RE database, but have different information in Sphere that prevents a match being made on an existing Constituent in RE. This can include addresses, phone numbers, etc.

Why do they need to be created?
They are created to keep continuity between your records in Sphere and your records in the Raiser's Edge.