Computer nowadays comes in different sizes. With advancement of mobile devices, some of these mobile devices are on par with full size computer. However, there are still advantage of building your own computer system
Yes, this is one of the advantage. You will able to mix and match your own hardware rather than what manufacturer think is good for you (Yes, I am looking at you Apple).
Hey if something broken, it is an easy fix. You can get the second hand hardware or you can buy a new one! Then just swap the broken part with the new ones.
Technology changes quite regularly and something you bought last year may well be out-dated now. With building your own computer, you will be able to be on the ball of the new hardware technology. So, you want loose to your mates again!