Saving to Hard drive on a Mac

I am looking to upgrade my computer at work which is a Mac Pro 5,3, and as part of that I wanted to take advantage of the 4 HDD slots inside, so that we work on some of our projects directly in the computer, without having to deal with an external HDD. My boss says that someone told her that it's bad to leave project files directly on the computer HDD, and that it's better to keep the projects on external HDDs instead. I can't really imagine why that would be, but I wanted to see if this sounded familiar to anyone, and if they know why that is, or if it's just rule that was made for computers where you couldn't add additional storage.

Thanks for the help!