suexec is used by the Apache HTTP Server to switch to another user before executing CGI programs. In order to achieve this, it must run as root. Since the HTTP daemon normally doesnt run as root, the suexec executable needs the setuid bit set and must be owned by root. It should never be writable for any other person than root.
For further information about the concepts and the security model of suexec please refer to the suexec documentation (http://httpd.apache.org/docs/2.4/suexec.html).