Updated: Sep 27, 2020
I can go to the local Walmart or BestBuy and buy a computer, pre-built, with all the cool RGB and best video-cards on the market, so why would I consider building a computer?
Fair question, easy answer... because you want to. There are plenty of reasons to build a computer but at the end of the day one decides to build a computer because they want to. Its something to easily customize, uniquely yours, and its fun...kinda.
Lets start with gaming computers, because...games. Now it is very hard to beat the big box retailers because of the discount they receive when buying in bulk, but you can build a system that is better or more purpose-driven. Typically, the computers at the big box stores hit all the keywords, i(3,5,7,9) XXXXk, AMD XXXX, GTX XXXX, and then they cut corners on cases, power supplies, RAM and little things that make a quality system really quality.
Unfortunately, most people who go out and buy these computers don't take the time to optimize their systems and they end up overpaying when they don't need to. For instance, why buy an "X" (for AMD) or "K" (Intel) processor when you do not intend to overclock? If you are just going to play some games you probably don't care about core frequency or overclocking, or TDP. So my suggestion, don't pay for what your not going to use. And you think "Oh, I'll just learn to overclock" but, if you're not an enthusiast, I doubt it. You are going to be greeted with a lot of frozen screens and black screens of death (even scarier than the blue screen of death) and general glitches (instability issues). Overclocking is not for the casual user, it's really for the hardcore, Not to discourage you, but you should know its not that easy. Also, most computers may have an over-clockable processor, but a cheap CPU cooler. You're going to have to spend some money on a new cooler, and what do you do with your old one? Next thing you know you have all these computer parts sitting around, and you overspent your budget. But now you can use the computer to its full potential.
Maybe you're not a big gamer, but you need to do some serious video or photo editing. Let me assure you that no computers at the big box stores are optimized to work these types of jobs, They can do them yes, because they are work stations, but you probably want something more purpose-built. Try finding a computer that does 3 way SLI at a big box store- probably not an easy find. This is where knowing what you're going to be doing, and building an optimized workstation, could save you some big money! With the right motherboard, proper cooling, an overclocked processor, and the right video cards, you can have yourself a major workhorse and can complete rendering faster than any computer in the big box stores.
Every computer is different, every workstation is different. Because of these differences each computer will have different potentials. Some will overclock easily, where others will require major cooling or power upgrades. It depends on the task and effort you are willing to put forward. Overall, I would always suggest building a computer, but that is because I am big into getting the most out of everything I buy. With a little effort and a little time, a purpose-built computer can offer you a better experience in almost every aspect of computing.