Firejail is a program that can prepare sandboxes to run other programs. This is an efficient way to keep a software isolated from the rest of the system without need of changing its source code, it works for network, graphical or daemons programs.
You may want to sandbox programs you run in order to protect your system for any issue that could happen within the program (security breach, code mistake, unknown errors), like Steam once had a “rm -fr /” issue, using a sandbox that would have partially saved a part of the user directory. Web browsers are major tools nowadays and yet they have access to the whole system and have many security issues discovered and exploited in the wild, running it in a sandbox can reduce the data a hacker could exfiltrate from the computer. Of course, sandboxing comes with an usability tradeoff because if you only allow access to the ~/Downloads/ directory, you need to put files in this directory if you want to upload them, and you can only download files into this directory and then move them later where you really want to keep your files.