I built mine for three reasons:
1. Apple wasn't making the model I wanted. I wanted something smaller, like a Mini, but with a DVD/BluRay burner because I still use discs. Someone either sends me a disc, or I make discs for others. Hey, my 93 year old mother in law knows how to use a DVD player. Plus I like redundant drives. I want options.
2. Apple hardware is expensive, and not upgradeable. So I can build my machine as I need or want to. I can use the DVD burner I already have and install the BluRay later on. If 16G of ram isn't enough now, I can add more later. Turn it into a server later on with a bigger case and four more drives. No problem.
3. I like to tinker and learn. It's satisfying to me to be able to build and troubleshoot my machine. At the tender age of 65 it's nice to know the brain still functions, can learn new things and adapt. Wait til you're there...