A pre-built system generally arrives in a "working" state. The manufacturer has done some due diligence and delivers a machine that will run. So arguably you don't have to install anything else, unless you are tuning, diagnosing an issue, trying to improve performance, etc. The disk is often provided mainly to serve as a recovery disk. I have run pre-built systems for years without ever updating anything. I was also a little surprised recently when I visited a motherboard manufacturer's website (can't remember which one) and they suggested not updating drivers unless you are having a problem.
Personally, my approach to a pre-built is as follows:
■ boot it up and make sure it is running
■ go to the motherboard manufacturer's...