One of the ways an attacker tries to penetrate an organisation is by emails. An attacker in a remote part of the planet can send a malicious email and start attacking an organisation. For obvious reasons an attacker would not want to be identified, therefore the attacker forges the sender address to cover his/her tracks. The victims whose addresses are being abused suffer the consequences as diminished reputation, or misdirected bounce messages.
You may have experienced this kind of an incident when you may have received a message that the message cant be delivered, when you have never sent that message!
Another attack that happened recently: https://www.republicworld.com/technology-news/apps/solarwinds-hack-details-another-email-attack-by-russian-hackers.html
Sender address forgery is a real threat that affects individuals and corporations.
Email is the most widely used form of communication but the confidence in this method diminishes with the ease of attacks using this medium.
DMARC solves the above problem and thus is aiming to restore the confidence with emails so that your bank can send you all kinds of confidential information which you can be rest assured has been sent by your bank and not by someone who is pretending to be your bank.