Sending emails is easy, getting them into the inbox and read is hard, especially when you are doing email marketing. With the prevalence of spam many mail servers now apply a credibility score to incoming messages and either reject the message, or send it to the spam folder unless it gets a high credibility score.
Domain-based Message Authentication, Reporting & Conformance (DMARC) is a monthly service that constantly monitors your domain reputation and blocks fraudulent senders which giving credibility to your mail server, web server, marketing service and another systems that send emails from your domain.
If you only send emails to clients and suppliers on an individual conversation basis from your devices then you probably don't require this service yet. If you use external services (Xero, Line-of-Business apps, Mailchimp, etc) and send messages in bulk then you need this service to get the message to the inbox.