"Some shelf registrations are done to cover deals that have already been inked, and some are done for imminent agreements."
Many are done to support an ongoing need for funding without any specific purpose/deal/agreement in mind. It's important to note that an S-3 of this type stays accessible for 3 years after it is declared effective and its main benefit is relatively easy access to funds on an as needed basis.....assuming, of course, that there is a reasonable market for the shares.