If you don't have DKIM setup for a domain, then your
email (not from address) will be signed with a Marketo DKIM signature. A DKIM signature header will be added to the email, which will reference Marketo (if you setup DKIM for your domain, it would reference your domain). Both are positive for email deliverability and will actually improve the chances of getting through spam filtering.
So, why is it better to have an email from marketing@domain.com signed with a Marketo signature rather than having no signature? Well, think of it this way. If a message is unsigned, the recipient email service does not know anything about the email other than the IP addresses that were associated with sending the message. With a signature, the recipient email service is now able to recognize the signature (even if it doesn't match the from address) and assign a reputation score based on this information. If, over time, they determine that most emails signed with this Marketo signature are "good" messages, then it is more likely these will be trusted over unsigned messages.
The signature will be invisible to your prospects as this signature is buried in the email's headers. If you are curious what a signature looks like, view the source of any email you've received from Gmail. All outgoing emails from many services (including Gmail) are DKIM signed by default.