Some tools only produce reports, while managed services walk you from monitoring to enforcement and keep third-party senders authorized. Compare how much help you get reaching p=reject.
Why it works
Enforced DMARC prevents direct spoofing of your domain to customers and staff, and major mailbox providers now require it for bulk senders.
What to look for
It should give you a clear path to p=reject enforcement.
It should discover every legitimate service that sends email as you.
It should alert you when a new source starts sending as you.