Why do we need to distinguish the two?

Doctors are all doctors, but there is a difference between a family doctor and a neurosurgeon. Lawyers are all lawyers, but a patent attorney is not a criminal litigation specialist.

It is fine to have a generic term for a broad profession, then get more specific if and when it matters.

There would be. Because someone who only knows html and css has never really encountered an if-else statement in there entire lives. It's really hard for me to consider someone who doesn't know what an if-else statement to be a "programmer".

Perhaps, though, I can still refer to these people as "developers."

Front end vs. back end.

Where does JavaScript belong then? I believe it's the most important thing for a front end dev.

Parent didn't specify, but usually on the front-end. I would disagree that it's all that important, certainly not more important than being able to lay out a page and style it without it turning into a mess. I've cleaned up crappy CSS work before, the only real solution is to throw it out and start fresh.

At my company where I'm the back-end guy, our front-end guy does most of the Javascript, but he has to get my help whenever he has to do anything non-trivial. Only a few projects a year ever has him needing my assistance.

Most companies' Javascript needs should be similar. Every once in awhile there will be a marketing push wih non-trivial Javascript needs, most of the time they can get by with basic jQuery. Hide this element, show it when there's an event.

Unless you're working for a news agency, making Snowfall every month, I'd be surprised if the business actually did need more. Of course, if the front-end guy decides he has to learn React, well, I'd say, go ahead, knock yourself out. But don't kid yourself into believing it's really necessary.

That's different. What do you call someone who knows html and css but no javascript? "Designer" might be the industry term.