Sure, you can plug a hard drive into a computer. But....
What about when that hard drive fails and you lose all of your data? Do you have backups? Do the backups _work_? Do you regularly check the health of the hard drive? What if you want to access the system from outside of your LAN? If it's accessible to the web, are you handling security yourself? How often are you applying patches? Kernel patches? What if you go on vacation and there's a power outage and you can't remotely turn everything back on?
What about when that hard drive fails and you lose all of your data? Do you have backups? Do the backups _work_? Do you regularly check the health of the hard drive? What if you want to access the system from outside of your LAN? If it's accessible to the web, are you handling security yourself? How often are you applying patches? Kernel patches? What if you go on vacation and there's a power outage and you can't remotely turn everything back on?