Here's a really simple demonstration of what the U+200B ZERO WIDTH SPACE does: it makes wrapping work in languages that don't have a spaces in their orthographies. To get the idea, try typing prestidigitation — it's too wide for the box, and you'll see a scroll bar at the bottom of the text area. (You'll also notice the annoying fact that your view keeps getting reset such that the "p" is at the left and you can't see what you're typing... but that's just my a bug in my dumb Javascript, nothing to do with spaces or anything).

Now, erase what's in the box, and try typing the quick brown fox jumped over the lazy dog (with the spaces). You'll notice that the spaces appear and then "disappear" — that's my script doing that — they're actually being replaced with ZWSPs. Your browser knows that it's okay to treat ZWSPs just like regular old spaces, and it will wrap accordingly.