Disasters can happen to your system or sometimes you need to read that disk that is formatted in another format, or whatever comes your way, sometimes you just need another system on your hardware and at that moment you can do with Ubuntu on a bootable USB stick. It allows you to install or upgrade Ubuntu and test it out without changing your PC configuration, or you can boot into Ubuntu and use tools installed by default on the USB stick to repair or fix a broken configuration, find lost files or read otherwise inaccessible files. Creating a bootable Ubuntu USB stick is super simple, and I have now a USB stick at hand always, just for emergencies, like reading Mac formatted files on a Windows PC or copying files from a Mac formatted Hard Disk. Bootable sticks can be made for Windows PC or for Mac. See the Ubuntu web site to learn how to create a bootable USB stick on Ubuntu (Note: other Linux systems are offering similar services).
(Image By Canonical Ltd. – Free Software Screenshot, GPLv3, https://commons.wikimedia.org/w/index.php?curid=131076905)