Its a VPN
Which is basically a way to connect to a network with encryption, so that all your communication goes through their network, kind of like adding a halfway house between you and the internet
It does not replace your ISP, and is usually used to connect to a company network securely, so that you can access network shares and print to office printers from home.
The practical upshot of this encrypted connection is:
All comms are encrypted so none can do a man in the middle snoop on your net traffic
If you are outside the US and want to appear that you are in the USA (say to watch US only broadcast video streams)
Your ISP blocks certain sites and you want to access them (say Pirate Bay)
VPNs are also popular in countries like China and Iran to get around web firewalls, and say expats Brits who want to use BBC iPlayer outside the UK, since you will appear to be in whatever country the VPN is hosted in