What kind of computer are you building?
For most (if not all) of you, the answer will be "one to play games and do some other stuff on," and that's perfectly alright. A gaming PC does a damned fine impression of a jack of all trades. They have enough CPU/GPU horsepower to keep up with Photoshop, transcoding/encoding of video, to watch HD video, and to web browse and use productivity apps without trouble. You might not be rendering a lot of 3D graphics (well, not quickly) but a well-built "gaming" PC will cover the vast majority of any user's needs.
But there are also other builds you might be looking at; supplemental, purpose built computers that will give you the ability to do things you couldn't or wouldn't want to do with your gaming PC. The thread isn't necessarily there just to help you build your latest Crysis-crushing monstrosity, there's a lot of knowledge on offer for other kinds of builds:
A Server/NAS can be a very basic 1 or 2 drive machine built out of an older PC you don't use anymore, or it can be a small form factor purpose-built machine that can hold 5 or 6 drives. It can even be a tower or rackmount build designed to hold 15-20 drives in hot-swap bays. And after you make the initial investment in building the server, additional storage is significantly cheaper than buying external drives (another common backup/storage strategy).
HTPC: Your entertainment center is the natural habitat of the HTPC (Home Theater PC). An HTPC can come in many different forms, from a small, low-powered, mostly passively cooled machine to a larger build that can incorporate full-sized components. Primarily, an HTPC is connected to your home network and the internet, so you can stream shared content on your network, or get your media fix via the internets. An HTPC can even supplant other devices if you include a Blu-Ray drive or a TV Tuner. If you build in enough horsepower, you can even use yours as a kind of game console. Naturally, you can use essentially any PC as an HTPC (just hook it up to your TV via HDMI), but a purpose-built machine will integrate better aesthetically with the rest of your A/V equipment, and may be quieter or otherwise better for use in your entertainment center.
Workstations: Computers for professional work (high-end photo and/or video editing, 3D graphics work, CAD, etc.) often have significantly different hardware demands vs. a run-of-the-mill PC. CPU horsepower is more highly prized, so this is the natural habitat of the 2700K, or basically anything on LGA2011. Depending on what you're doing, you might want specialized video cards designed for 3D modeling, high end rendering, or even GPU compute. Workstations often vary in component choice significantly based on what you need, so you'll want to spend some time in the thread narrowing down your needs and what hardware will work best for you, but the end result is that you won't have to overpay for a Mac Pro.
If you have any interest in a build like those above, or maybe something even more esoteric, please join us in the thread!