I’ve only been using Blender for a few months and every tutorial featuring a world texture says to use a HDR image, but none say why. I’ve been using regular equirectangular PNGs so far and getting the desired lighting effect, so I’m just wondering what benefit HDRs provide? Cheers.
a HDR (high dynamic range) image will give you more information about the light and the intensity of that lights.
as you look around you, you can see that the sun is much brighter then a lamp. in an HDRi these intensities are stored and used to light the scene. in a LDR (low dynimac range) image the max brightness is at 1 for all lights. there is no difference between the sun and another lamp in the scene
Thanks for the info. I’ll have to look into getting some HDR panoramas then 
If it helps: consider a render-output (or any sort of file used in the process of rendering) to be what it actually is: “a file of computer data.”
The data requirements for such files are much, much more demanding than those of, say, the slide-show gadget on your boss’s desk, which endlessly displays pictures of his kids, his wife and his dog.
Although, at the end of your project, you might finally produce “an image-file” (.JPG, .MOV, what-have-you) as a deliverable which contains (and which, at that point, needs …) “nothing more than that,” you very much need “high dynamic range” data during the process of creating it. Yes, you need “whiter than white,” and “blacker than black.”
Otherwise, what happens …? You guessed it: [data] loss. “Loss” as in: “gone, kaput, bye-bye, can’t get it back.” Not good. You must only “throw-away information” when you are good-and-ready to do so. (And then, only because you’ve still got it.)
Best is to start where all things on the matter are explained 
Smart IBL (Image Based Lighting)
Also check the pioneer, the master and his works… Paul Debevec Home Page