You want to lift big weights. We want you to lift big weights too. That's why you have probably heard one of us barking at recently you to take in a half breath and hold it during the lift. This is because we want you to maintain a stable and neutral spine so you can both recruit the most force as well as protect yourself. Holding your breath makes your torso more rigid, applying pressure on your diaphragm and creating a stable, functional, lifting machine.
If you are having a hard time understanding this, imagine for a moment that your trunk is like a can of Dr. Pepper (why Dr. Pepper you ask? Because if you are going to drink a soda, Dr. Pepper is the best). When you are maintaining your half breath, you are like a full closed can. You can't bend it, and if you try to smash it, it will more likely break your ankle. This is your trunk pressurized and stabilized by your held breath. When you breathe out, your diaphragm no longer applies pressure to your trunk, and you lose stabilization, much like the empty opened Dr. Pepper can. That empty can is both bendable and smash-able. Under a heavy bar - which would you rather be?
Breathing is always an important part of technique, no matter the exercise. By holding your breath during those heavy deadlifts, squats, cleans, etc. you will have a more stable torso and spine throughout the lift. A word of caution though, this technique should be applied to lifts individually, so you don't make yourself pass out.