I blame the Kardashians for the contouring trend: those ladies really know how to define their cheekbones. The reality show family may have put the makeup technique on the map, but the world definitely ran with the trend: heck, just read up on "vagina contouring" to see how far this contouring thing has gone. Now we have a way less weird thing to contour: our hair. Hair contouring is a thing too, and while it may lead to pretty highlights, I'm not sure it's worth it.
So what is hair contouring? According to stylists, the endgame isn't that different from when you use makeup to contour your face. Hair contouring places highlights and lowlights in different areas of your hair to give the illusion of a certain shape. If you have a longer face, for example, you could put some strategically-placed highlights that frame your face and therefore give it a wider appearance. As with makeup, darker colors are for shadowing while lighter colors highlight the areas you want focused on.
Here's a picture that colorist Nick Penna pointed out as an example of hair contouring for Allure. Does SJP's hair accentuate her best feature?
While I'm all about playing around with makeup contouring, specifically designing your hair so that your face looks a certain way doesn't seem particularly necessary. The trend may work when your hair is down, but what happens when you flip your part or put your hair in a ponytail? If hair contouring allows you to score some beautiful highlights, I'm all for it, but ultimately, the point should be to have pretty hair - not have pretty hair that accentuates your cheekbones.