no, the contributor group is a sample group exemplifying typical access rights for authors on an author system. qouting from the documentation:
"Basic privileges which allow the user to write content (as in functionality only).
Does not allocate any privileges to the /content tree - these must be specifically allocated for the individual groups or users." 
you can safely delete the group or revoke its ACLs. with time you're going to create your on author groups once the understanding of the required ACLs deepends.
the admin user (or members of the administrators group) always have access to all nodes.