We use it as a corporate or as an exchange email filter to protect, to stop any executables from coming through email, and to halt phishing scams. Basically, the product is for email security. We use it for our network.
Also, if a user thinks that they didn't get an email, we can access the Proofpoint dashboard and go find missing mail that either people didn't get so we can rerelease it to them. Or if it did get quarantined, we can release and approve a sender.
You can blacklist and whitelist sender lists. If you are getting phishing scams that get through the filter, you can end up shutting those down. You can actually block the entire domain so that domain can never email you ever again.
If there is an external email outside of your network or your company, you can whitelist them if it's an unknown sender, so you don't ever have to worry about missing their emails.