Tanking successfully often hinges on control -- control over your surroundings, the enemies around you, your damage intake, the threat ceiling, and so on. The more drops of tanky goodness you can squeeze out of your every one movement, the better. And that means that the more efficient you are with your GCDs, the better your performance will be.
One of the easiest ways to make your actions more efficient is with the judicious use of macros. No, I'm not talking about the heresy that was the oft-paired 9 macro and 6 macro back in Wrath, but rather some neat little tricks that can give new forms to your abilities, prevent disastrous side effects from others, give you better raid utility, or even allow you to dominate the interrupts chart on a certain fight.