I have never built a handgun, but I have done a couple AR15s.
Most of the internet gun forum reasons for building them, never entered my mind. Saving money wasn't part of the equation. Having a lower receiver with no paperwork wasn't a consideration.
I had put together probably a dozen AR-15s, mostly the same ARs reconfigured every five years or so. I started out of course installing a few minor after-market parts, then worked up the nerve to assemble a lower which wasn't difficult at all. Then finally, years later, bought the tools and assembled a few uppers, which also was not difficult at all. So, I thought it would be fun and interesting to finish an 80% receiver. Just to take very simple gunsmithing to the next level.
I am not mechanically inclined. I can see what needs to happen, I have the tools to do it, but often when I try to do whatever it is I screw it up. The AR15 allowed me to dabble in gunsmithing to a small degree and I enjoyed it.
Finishing a lower is just another fun project that you can then later use and proudly show off as something you did yourself.