Additional insured status extends a subcontractor's coverage to protect the general contractor for liability arising out of the sub's work. If a third party sues the GC over something the sub did, the sub's policy can defend and indemnify the GC.
This is the protection at the heart of risk transfer, and it requires an endorsement, not just a check box on the certificate. Confirming both ongoing and completed operations additional insured status is a core COI review step.