One of the biggest questions that often arises in Olympic lifting and now Crossfit is when to use a weightlifting belt.  Is there an appropriate time to strap it on or should you ever use a belt?
Before this question can be answered we must first define three terms as it relates to Olympic lifting.  Take these with a grain of salt as I am defining them personally for the sake of understanding when to use the belt.
Competition: The element of Olympic lifting solely focused on performance as measured by the most amount lifted under strict standardized time constraints and unbiased judging.
Testing: The element of Olympic lifting by which competitors stretch the limits of their capability through heavy attempts and near maximal or maximal lifts.  This is always performed as part of workouts performed in a training environment.
Training:  The element of Olympic lifting in which the lifts are practiced and completed at sub-maximal levels focused solely on precision, volume, strength gain, and perfection of movement rather than the amount of weight lifted.  The goal for a training day without testing is always perfect movement even if that means the percentage of maximal lift is limited.
The belt has specific times it should be used depending on what phase of lifting you are in.  I NEVER use a belt during a training phase.  Primarily this is because I don’t need the extra midline stability for the amount of weight I am lifting AND training without a belt helps develop my midline stability and make me a stronger lifter.  Since the goal of training is strength gain and perfect practice of the lifts then it only makes sense to take away the belt.  I generally classify any training lift as a lift at 90% or lower.  I never wear a belt when I am lifting at that percentage range.
In the competition phase its obvious that a belt must be used.  Since the goal for competition is to win and post the highest total possible then it only makes sense to do all you can legally to gain an advantage.  If the belt helps you lift even 1 kg more then wear the belt.  Competition is about performance and reaching your maximal potential.  The belt helps achieve that in the same way smelling salts or competition heel inserts do.  Use all in your power to legally achieve the highest total possible.
The tricky phase for considering when to wear a belt comes during testing.  What is your goal for testing?  How often are you testing?  If you are in a phase of your lifting where everyday you step in the weightroom to test a different complex or a different movement then I would urge you to leave the belt in the bag.  Relying on the belt day in and day out will make it a crutch and a mental safety net that is unnecessary.  There are periods in my training during the past where my coach has taken my belt away altogether regardless of what is on the bar in order to prevent this mental dependency and also to develop midline and core stability.  However, if you have specific days where you test each week or each month then those times are likely the appropriate times to strap on the belt.  If you are testing your 1rep clean or front squat at the end of a 12 week cycle then certainly put the belt on.  When you wear the belt as part of your testing you have to consider both its capability to help you lift a little more because of stability but also the dependency that can be developed if you ALWAYS wear a belt.
As a sidenote, I generally do not recommend wearing a belt during the snatch EVER as the barbell can hit your belt on its way up the body.  The nature of the snatch creates a barpath where the belt can become a detriment to bar speed.