Why is the CV needed for graduate application
A CV is the academic equivalent of a resume, providing all of the relevant details of your (academic) career to date.  Graduate schools ask for this information for the same reason that businesses ask for a resume when you apply for a job: it is a terse summary of your qualifications as a candidate.

In addition to publications and (research-related) positions held, a CV also should include your undergraduate and graduate degrees, service, any awards, and any other academic-relevant information about you.  A nice summary, with links to additional guide material can be found here.