Well, first and foremost, Sandstorm uses Cap'n Proto because Cap'n Proto was built, first and foremost, to enable Sandstorm. :) Though of course, now it also powers Cloudflare Workers and a bunch of other things.
My favorite explainer on the object capability model is... the first half of this exceptionally long page: http://habitatchronicles.com/2017/05/what-are-capabilities/ It's worth the read, but read it when you have time to sit down.
So access to apps or documents by users, as well as those apps and documents being able to reach out to other resources, is all generally wrapped in capabilities.
I think as a self-hosting platform, the biggest advantages of Sandstorm is that we force developers to provide a no-config-file, no-sysadmin-required experience, so Sandstorm is probably the only self-hosting platform currently intended to be used by people who are not tech people. This naturally flows into some other things: If you don't have a sysadmin running it, it needs to be really secure (capabilities) and very good at self-managing it's own performance.
So Sandstorm apps do not consume any resources unless you are accessing them. Everything is spun up and closed down on demand. And the platform's security model not only means vulnerabilities in apps mostly aren't exploitable and apps don't need to be patched, but Sandstorm strives to safely allow non-technical users to install and run potentially even malicious apps without risk.