And yes, if you're adding 5+ GB to each image build through something silly like a 'RUN chown' it's worth fixing. Maybe not complex refactoring, sure, but we are talking about the basics here.
This Docker and Kubernetes hype and the fact that few people understand and care to implement these technologies correctly fills Amazon's and Google's pockets with money that could be spent elsewhere.
Learning layers is not critical to get up and running with Docker. You can learn it easily afterwards, but on the very first introduction it's just unnecessary cruft that distracts you from your goal.
More importantly, you can actually use Docker in daily practical applications your whole life not caring about layers. Think about it: why should you care about layers if you don't care about build size or build time?
At this stage, caring for layers is just premature optimization.