With this build, I am just buying the lower receiver, assembling the lower parts kit and stock assembly, and I'm going to buy the upper. You can build the whole thing and have a completely custom rifle by picking your upper receiver, bcg, barrel, etc. Some guys even buy a milled lower receiver that's 80% (or less) completed and finish the milling themselves, usually to avoid either FFL fees or registration. I'm not building my upper because it requires tools i don't have and has some steps i'm not entirely comfortable with doing myself, but they aren't that bad. Maybe i'll do that with my next build.
If you assemble the rifle yourself:
1. its cheaper
2. you can customize it, which most ppl would do with a complete rifle anyway
3. satisfaction doing it yourself and knowing you have a skill 99.99% of ppl don't have
I decided to build my own, mainly because I wanted to learn how it works internally, gain a skill, and as a hobby. IMHO, if something breaks, I like to fix it. In order to do that, I have to know what broke, how it works, and then what to do. And when the Zombie Apocalypse happens, that's going to be important.
This build is going to be a basic, non-tacticool, high quality, reliable, "bug-out" rifle. If I were to buy a completed rifle configured the way i want, it would cost about $2000. Building it myself, it'll cost around ~$800.
