If you already have a PC you probably have no real reason to get one as you can already upgrade your existing PC for less (or more power) by reusing some parts. I suspect this is more for people that are looking to get into PC gaming or are currently using laptops.
Also, its just a PC. Loads of companies make PCs. I worked in one before, we had a staff count of 2 and the PC building was part time. Obviously valve have greater R&D funds than we did so they can afford to make their own custom form factor.