I am a learner, and I currently use it just to stay sharp because my current place of employment uses it (they're actually still on Bootstrap 3, for some reason! Huge relatively prestigious university.) and if I was to move to the Office of IT, I would need to know it. A lot of local job listings also request it. It helps when media queries are grinding my gears and I don't want to put a ton of effort into them.
That's true, since it's so popular it's useful to be familiar with it for jobs.
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.