Building your own pc is relatively simple. Find the right components that work together, purchase them, and then put them together in the right order. There a great deal of benefits gained from this over that of buying one pre-assembled.
First, it allows for specialization and customization. You may need a computer for simple tasks, or you may need it for high end gaming, or some level in between. By picking out the parts yourself, you can make a pc that suits your specific needs without all the other components cluttering up your system or costing you money.
Second, building your own pc allows you to design one that will let you upgrade it relatively easily down the road. You can choose the components that will work well in newer models. Say you only need a dual core processor today with 4 GB of memory, but you want the ability to install a hexa core processor and maybe up to 16GB of memory. By building your own pc, you can plan for that, rather than have to buy a completely new computer when your needs increase.
Third, performance. By building your own pc you can create a performance machine. By customizing and upgrading your system you can tweak certain areas to find out what part may be holding your system back and then upgrade it, giving you greater performance.
In conclusion, it is a good investment to build your own pc. As you upgrade parts you can retain the old parts and use them in older machines. There is also a broad range of other people who have built their own computers and can provide tech support should you need it. It is a lot simpler getting help from someone who has built their own computer rather than waiting on the phone for some IT guy. Thus, with the specialization, customization, and performance increase you can create a truly versatile machine that will last far longer than a typical store bought computer.