The resume database allows employers to look at the resumes of those looking for work. The point is, job seekers have another opportunity to put themselves out there without necessarily having to apply for a specific job.
The "Apply" section of an actual job listing does not require a job seeker to be registered to apply, so not all applicants will have a resume loaded up on JobRoller. Also if you consider the employer receiving the applications, they may find it more convenient to receive the resume directly in their inbox rather than visit a URL.
