Building your own computer isn't as hard as it seems. You just need to consider what you will use the computer for, pick components accordingly, and you can avoid paying extra for accessories or performance you don't need.
If you have a lot of movies, music or images, you'll need a lot of storage space. If you're a gaming enthusiast, you'll need a powerful graphics card. If you're into picture or video editing, you'll need a lot of RAM memory and a good processor. Some people want all of this, while others just need some of them.
You can also upgrade an older computer with new parts to improve performance.