Personally, I never bothered having an extra theme installed... until site health within WordPress required it. And, in all honesty, my other tech team had to install a backup theme while troubleshooting, so I understand the value of having that extra "insurance" ready should you need it. :-)
You're very welcome Erica, site health now suggest having one theme as a backup should our default theme fail. This way we will have a theme to use while troubleshooting.
Hey Matt, thankfully you can always re-install your backup theme. Site health suggests that we have a theme ready to use should our default theme fail us. That way we would have a theme to troubleshoot with.