Depends on exactly what "from scratch" means. It could just mean "I don't trust what the manufacturer pre-installs and intend to reinstall immediately", which seems to be good advice on Windows and reasonable advice on Linux distros. Like... I wouldn't necessarily advise hand-configuring everything starting from a netinstall shell, but I do set up my systems by running a vanilla installer to lay down the base OS and then running my ansible playbooks against the system to make it fully functional; does that not count as "from scratch"?

Knowing whats on it, configuring it the way you like, selecting a different distro you prefer, certainty that if you needed to you actually could deviate from the OEM install... There are plenty of good reasons to load your own OS on your own hardware, that aren't purely for the exercise.