Totally confused about Sandbox

[CaptSaltyJack]

Hmm, ok. But a regular SuperDuper backup can also act as a sandbox. If I'm wary of something, I can just boot the backup and install whatever, to see if it works. Sandbox seems like the exact same thing as a regular backup, except that user files & apps are shared from the Mac HD to the Sandbox in real time, correct? Question: if I boot the Sandbox and install an app, it's not shared back to the Mac HD, right? In other words, it's a 1-way share?

Ok, so basically I still use the Mac HD most of the time, I can just use the Sandbox volume when I want to try something I'm wary of. The user guide kinda makes it sound like you should use the Sandbox volume all the time, which threw me off. In case you're curious, this is the part that made it sound like the Sandbox becomes your full time boot volume:

With SuperDuper, you actually use the Sandbox as your startup
volume.

You can safely install any system updates, drivers or programs in the
Sandbox, without worrying about what might happen to your system. If
anything goes wrong, you can simply start up from the original system.
SuperDuper has preserved it in its original, pre-disaster state – but all your
new and changed personal documents are totally up to date.

Especially the part about "but all your new and changed personal documents are totally up to date." Why would I be making a bunch of changes to my documents on the Sandbox volume if I just booted into it to test out an install? That makes it sound like I'm actually doing real work in the Sandbox, and it's become the volume I boot into to get stuff done. Even worse, what if I ran something on the Sandbox volume that was malicious and wiped out a bunch of my personal files? Now those files are missing on Mac HD? Of course that's probably a rare scenario, but still.

This is also confusing:

Of course, the original volume doesn’t have any of the programs or system
updates that you might have installed since you made the copy. That’s a
good thing, since they’re probably what caused the problem in the first
place!

Why doesn't my original volume have programs I installed? I thought apps and user files are shared? After all, above it says "all your new and changed personal documents are totally up to date" [on the Mac HD main drive]. So if I chose "shared users and applications," then potentially bad apps from Sandbox would also be shared over to Mac HD too?