A horse should technically be able to do all levels in a snaffle, which is why the FEI recently allowed it an option to ride in a snaffle in the Prix St. George through the Grand Prix levels.
Some riders will only ride in a weymouth bridle in the show ring to ensure that the horse sits down properly when it should, a little extra 'oomph' to the rider, if you will, since odds are the horse (presumably and more often than not, the horse is a large, athletic warmblood) is a little distracted by all the sights and sounds and there's a lot of investing (money and time-wise) done in competitions. It is also used occasionally at home to 'sharpen' up the horse a little, but a good rider will never use it more than occasionally and for this reason, It should also only be used by experienced, sensitive riders who know precisely when to apply the curb.
It's a fine, effective, forgiving and kind tool when used by the right rider.
Unfortunately it can be abused and it's effectiveness taken advantage of, as well. I have met my fair shair of upper level horses that have had two bits shoved in their mouth met at the end by hard, unforgiving hands. This, in return, makes the horse unsensitive and hard in the mouth. The opposite of what you want/an unpleasureable ride. Not the horse's fault, of course. Just too bad.