Sentry is remote control for your website.
You've built a fantastic website and are out celebrating with friends. You take out your smartphone or tablet to show them what you've been working on, and realize there's a problem. Maybe an image is wrong, or the RSS feed being displayed on the homepage is broken.
You can't show it to them like this!
No worries, because you've added Sentry to your site. You quickly login, hide the offending stack, then preview to see that everything is looking just how it should be.