Reason number 1 is Privacy
Reason number 2 if I don't want it and if I can disable an app I always do
For example I always uncheck in all my apps "Check for new updates"
If the macOS intaller allow us to choose what apps we want to install, I wont install some of them that came by default
for example I never install Siri, I don't need it, I don't use it...
Maybe for other users Siri is the perfect app in their computer but not for me
That is why I want to know If I can disable Spotlight
Including all your HDD's in the Spotlight Preference Privacy panel effectively disables it, bear in mind that once disabled the built in search feature for things like Finder and Mail will no longer work.