If you are taking pictures of all the partying that goes on around the campsites, yes it will be useful!
However, it isn't really that easy to shoot the cars at Le Mans, there are only a few places not behind a fence and there will be quite a few people all trying to shoot from there. If any of the cars have reflective stickers the flash will pick them up, but you will not be able to light a whole car from a hotshoe flash, my advice is to go and enjoy the raving and partying (and take good waterproofs).