What is the Value of using DKIM to sign mail when the from header doesn't match?

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http://serverfault.com – My domain is domain.tld, where I have DKIM functioning just fine. All of my domain emails are signed, and pass. I have a need now to send mail on behalf of my customers occasionally - ie: send email from [email protected], to [email protected] I know that I can sign the Email with my domain.tld DKIM key, and it will "pass". But really, any spammer could do this as well, so what value does this provide? So, is there a value in signing an email that I'm knowingly "spoofing", on behalf of my customer? It seems like there must be because, this is a pretty common practice, but it (HowTos)