I have both Firefox and Chromium installed on my computer.
Because what if one of them royally forgets up and is unusable?
I have more than 1 browser installed, but I only use one.
The other is only really as a backup, but I've never encountered an issue where I'd need it. If there were an issue that would properly prevent me using one of my browsers (like lots of viruses) it would probably of such magnitude that I'd need to factory reset the computer anyway.