Application 1: Extending an existing HDMI Cable
There's always times when an existing HDMI cable is just too short for the job at hand, and while in most cases we'd reccommend just getting the right length cable, an extension cable can be handy for adding an extra few metres in most situations. Just be careful when using HDMI extensions on already long HDMI cable runs. Just because you might be able to get a 1080p/60Hz signal working with a single 20m HDMI Cable, that doesn't mean a 15m + 5m extension cable will work to the same ability (remember, there's signal loss every time you add another join a cable run).
Application 2: Adding accessibility to an obscured HDMI socket