Primary and noncontributory is a two-part requirement. Primary means the sub's policy responds first to a covered claim; noncontributory means it will not seek contribution from the GC's own insurance. Together they keep claims off the GC's loss history.
Without this language, both policies might share a loss, raising the GC's premiums and eroding their limits. It is typically added by endorsement and confirmed on the certificate alongside additional insured status.