According to the recent NSSF MSR report, everybody and their mother bought an AR-15 in the last two years. Many of them are first-time buyers, who might not have a lot of experience cleaning their new toys, or which parts are the most crucial to clean.
This vid does a good job of showing you what you need to know.
Side note: I’ve owned a couple of ARs, and I’ve never seen an extractor spring doughnut like Sootch uses on his rifle. Is that common?