Why do ESP forwarders benefit from ARC?

Mailing lists such as Google Groups and similar services are the primary beneficiaries of ARC. Their role involves receiving mail and resending it to many subscribers which breaks both SPF and DKIM and causes DMARC alignment to fail. Before ARC domains at p=reject often could not be used safely with mailing lists. ARC allows these intermediaries to sign the message as a trusted forwarder preserving the original authentication and proving to the final receiver that the message was legitimate before the list modified it.