The short answer is to distinguish titles from the main body of text (or synopsis, lede, etc.)
It doesn't need to be very readable because it's used sparingly, and it's more important that it stands out so you know what you're reading, etc.
How to Get Attention and Keep It.
The untrue story of an internet troll who thinks the damsels are lost and knights assholes trying to take advantage. by some guy whose definitely not the troll.
This has been a thing for centuries at this point, though it seems to be falling out of fashion. You can find it in style guides already just after the turn of the 20th century and the practice certainly dates back further. Some publications online have stopped doing title case in favor of sentence case, but I'm not sure in physical media. Title Case sets the title apart, may be clearer or catchier at a distance (good for newstands and newsies, I guess?) and I find it easier to read in some cases. Some argue it stems from German which capitalizes all nouns, but I doubt that since this is done everywhere in German and not just in titles.
It's certainly not a universal thing either, no other language I know does it.
Once I asked on linguistics (or maybe English language) stackexchange about the origins of it, but it got deleted as duplicate of a related but definitely different question. Most satisfying stackexchange interaction