The great thing about the 80% lowers, regardless of plastic or aluminum material, is that anyone with an once of DIY in them can make one. Tolerances while best to be exact quite honestly can be off some and the weapon function fine. Taking your time, a few simple and very affordable tools (you don't have to spend more then 100$) and anyone can make one.
Most people initially think it's about saving money which it certainly isn't. Others think it's to have fully unregistered firearms and I guess that's a personal thing though not for me. I like tinkering, working with my hands, being creative and completing a project that's unique and when finished I can say, Made By My Hands.