If kill-switched option is activated and that, for any reason, you get disconnected from ProtonVPN, you internet connection will be blocked so the only internet connection you have will be secure, even if there are some trouble.
If you want to exclude some apps from the VPN (for example if you don't want a social network to know you're using a VPN), so all your connections will pass through ProtonVPN except traffic from the apps you excluded.
Even in case of DNS leak, your online activity can't be leaked because ProtonVPN is also protecting your DNS requests.
ProtonVPN is available for Windows, Mac and linux !
A Mobile App is available, for Android and iOS.
You can have one account for several devices - for your family for example
If you're connected to ProtonVPN these entities won't be able to block or censure you; they won't be aware of what you're doing but according to the amount of data you're consuming they can guess: they can know if you're downloading things (but they won't be able to determine what), watching movies (but they won't be informed of which movies), or browsing classic websites.
This is more like a commercial sentence : nowadays almost every website is secure (thanks to https) so that password can't be stolen by any intermediary between your device and the website you're browsing. This was a good argument a few years ago, now it's pretty useless.
If some websites or services blocked your IP you can bypass it thanks to ProtonVPN.
If a website (Netflix, Youtube, ...) doesn't offer some services (like movies) in your country you can also connect yourself to an american server and therefore access to the blocked content.
Finally if someone want to d(d)os (try to block you internet connection) you, he won't be able to do that, because he will attack ProtonVPN IP, not yours.