Simply put, a sandbox provides you with:
Nearly instant access to a computing environment of your choosing.
A set of interface components that allow you to interact with that computing environment (such as a terminal and code editor).
File persistence so you can return to your work later (even years later!).
A sandbox is designed to help you with two things:
If you need to write some code or use a piece of software that's already in a sandbox, you don't need to worry about installing anything, and can focus on the task at hand.
If you want to try a new piece of software without cluttering your local environment, you can first install it in the sandbox and try it there before committing to installing it on your computer.
Both of these use cases are explained further in the next section!