Organizations and publications often have to receive documents from outside people for many reasons. Magazines accept article submissions from freelance writers. Academic journals receive papers from scientists. Companies receive resumes from jobseekers.
These documents need to be reviewed by the organization and then decisions need to be made - should the article be published? Should we interview the candidate?
Submittable makes it easier for the outside people to send the documents, and makes it easier for the organization to receive, review and decide on the documents.