For me it's to build a rod exactly how I want it. I can pick and choose the best blanks that each company makes and use the exact components that I like a rod to have. I can't buy a factory rod set up the way I like it, with the guides, reel seat, hook keeper, keeper placement, grip, grip placement, etc. exactly how I want. I can build one pretty easily though.
Some people like to have something with custom colors and a custom design, that's more important than the performance aspect, or a combination of the two. For some people it's just something rewarding and creative to do that keeps you involved in fishing. It's cool to catch fish on a rod you built.
I never found money savings to really play into it at all.