I used to apply tourniquet during my initial phase of IV experience. It used to make finding veins much easier and being much younger and ignorant, I thought that I was supposed to use it. Nobody showed me how to use needles, shoot drugs; I pretty much decided to shoot coke one day when I was abusing any substance around me heavily. None of my friends used this route so I just started myself and learned as I go other than very essensial points such as not shooting into an artery. I slowly figured out more about the correct dose, needle size and way to tourniquet. Then, once I got some experience and more knowledge about the mapping of my body, I stopped using it. After a few years of going deeper into IV drugs (coke, heroin, morphine, MDMA, meth, speed, diazepam, ... and soo stupid but even LSD), I became a pretty hardcore junky for lots of years during which I had to use tourniquet in order to find my deteriorating veins.
But usually, once you get better at injecting and learning your body, I don't think that there is much need. The worst use of tourniquet that I saw was a kid choking himself in front of a mirror in order to shoot his vein.