So, if your jailbroken iPhone ever ran into any software issues that prevented it from booting up, and you did not have OpenSSH installed, you would be forced to restore to a stock firmware, thus causing you to lose your jailbreak. However, if you had OpenSSH installed, you could simply connect your phone to your computer and try to resolve the issue and hopefully fix the device.
Additionally, CoolStar's SemiRestore has just been released for Windows (Mac coming soon) with iOS 7 support and it too requires OpenSSH to be installed beforehand. SemiRestore lets you restore your device to a clean state, without losing your jailbreak. It is not a full iTunes restore, but does erase all content and settings. SemiRestore also has the ability to repair system files if they ever become corrupt and prevent your iPhone from booting up. However, this too requires OpenSSH to be installed beforehand.
A common misconception among jailbroken users is that OpenSSH causes a battery drain. However, since OpenSSH is simply a launch service, it does not affect battery life as it only runs when you are connected to a device. We strongly recommend that you change your SSH password for better security (the default password is alpine).
As always, be sure to only install packages from trusted developer and repositories! However, if you ever run into any issues, try holding the volume-up button on your device during boot, which prevents disables MobileSubstrate tweaks from running, allowing for a safe-mode boot. If that doesn't work, you can always try SemiRestore or use OpenSSH to try to fix the problem yourself.
For those who don't know how to install OpenSSH, you can follow this tutorial for instructions.