A random VPS service (preferably in another country) only cares about you insofar as you pay them and don't cause any trouble to them. They don't have as much of an incentive to invade your privacy as your home ISP does, and I trust incentive structures a lot more than I trust boilerplate words on a privacy policy.
It can also be a matter of opportunistic encryption. Most public wi-fi is vulnerable to anyone in the vicinity, in addition to the usual ISP and the NSA. Use a VPN and now you're only vulnerable to the VPS service and the NSA. That's quite a bit of improvement.
You also have the freedom to choose a VPS service with good connectivity in a relatively less snoopy country, a luxury you often don't have in choosing your home ISP.