When you deploy from the marketing system, you will likely want to use a "from" email address attached to your domain. But the marketing system does not exist on your domain and recipient servers can see where the email comes from. It would be like receiving a letter where the return address says Las Vegas, but the postal mark says New York. When the stated "from email" (return address) does not match the "from server" (postal mark) it looks like the email may be spoofed. The email may be rejected because it seems shady that the sender is attempting to hide the origin of the email.

We know your email is legit. You know your email is legit. How do we show recipients that it's legit? By adding the SPF record to your domain, you are giving the marketing system public permission to send on your behalf. The recipient then has the ability to authenticate your emails by verifying that the marketing system has permission to send emails from your domain. This will decrease the liklihood that your emails will be rejected or filtered as spam.