Postmaster@ and abuse@ accounts are normally used by other postmasters or individuals that run corporate email systems to report issues with your domain.
Let’s say one of your users in Marketing is sending out a mass mailing and, well, didn’t ask anyone if they wanted to be on it. Instead of the person who runs the recipient’s mail servers posts your domain to RBLs, they could email you at abuse@ to inform you that this is occuring.
Additionally, if someone is getting nondelivery reports when sending to your domain because they are listed on an RBL, they should be able to send an email to postmaster@ and let you know that they’re not trying to sell you anything nor did they get 15 million from a dead relative that they want you to help them bring into the country.
Now, I would suggest create an account called postmaster, give an alias of abuse, and simply write a fwding agent to all the admins from that account “before new mail arrives” to let them know that someone’s trying to tell them something.
HTH, and if I’m wrong on any parts, please let me know!