On a micro scale, your job clearly isn't worthless. A large tech company needs patent lawyers, or it will be completely destroyed by its competitors. But what if all patent lawyers suddenly resigned and started their rock bands, or became carpenters?

So, "bullshit jobs" are essentially jobs where you are playing a zero-sum game. Like SEO: your site goes up, another site goes down by the same amount. And you have to keep spending on it, otherwise your site will fall off the first page.