Is it necessary for that price? Are you asking if the price difference between buying parts and building it is much better than buying a preassembled computer?
Well that depends, on a computer in the $1000 price range, I'd say the difference isn't huge, you wont be saving a whole lot of money. The real benefit of building your own computer is so that its parts can be specifically tuned to your needs. A lot of preassembled computers aren't built for gaming. Depending on where you buy the computer, for $1000 you can buy a great computer (For instance, buying at a computer hobbyist store), or you can get a terrible computer (For instance, buying at place like walmart).
Another thing to consider is if you really know how to build a computer. It isn't rocket science, but you do need quite a bit of knowledge about computer parts. First, you need to make sure each part you buy is compatible with the motherboard, you also have to make sure you get a proper power supply. You might find out half way through you can't build it yourself and have to pay $100 for someone to finish building it for you. In that case, you'd likely lose money.
The reason a lot of people build there own computer is simply because they find it fun. I built my computer about a year ago, and to tell the truth, I didn't save much money apposed to if I would have bought preassembled. I could have if I went bargain/sale hunting, but I didn't.
Not sure if I even answered your question, I'm not quite sure what you're asking.