What is pfsense really used for ? I see it being used to replaced ISP routers, I see it being used as routing vpn traffic, but is there one or two majority uses for it ?
It’s primarily a firewall which by nature is a router too. Most firewalls have VPN capabilities as does Pfsense, but it also has packages which allows you to install monitoring and security tool amongst other things.
If you go to their site you will see their marketing on their front page:
So clearly they want to be associated with “network security” in general more than with the firewall aspect which is their origins.
NetGate, who makes pfSense, sells hardware preloaded with the software. They sell tiny little boxes for use at home and they sell larger boxes with many ports and powerful CPUs for use in a large company (or ISP.)
They use a Unix based OS (it was FreeBSD I don’t know if they changed recently, I think there was some discussion they might.) They have a web based front end for user configuration, but you could theoretically do all the management from command line (if you were a CLI maniac.) They also support the ability to have add-ins. These add-ins can do all sorts of things, from VPN to filtering. If you wanted to, you could presumably run your own code from a SSH shell and use it as a general purpose computer, but that might impact its core mission to route (or block) packets coming in or going out of your network.