There's a wide spectrum of personal preference for internet privacy, and if you're the type that doesn't trust your ISP you could potentially use TOR to add an additional layer of anonymity. If you trust your ISP there isn't much point.
http://www.theverge.com/2013/7/21/4541342/isp-owner-describe...
While your message contents would still be encrypted when using the regular old ProtonMail site, it would expose your identity and who you are sending messages to.