I've never been to one either - my local counter has nothing like this. But from what I gather it's a chance for you to go and view and buy the product collection early-ish.
You will be able to get a 30 minute mini makeover using colours from the collection. Apparently this is just eyes/lips/cheeks so you might like to go with your basic face on already.
Other than that there might be some little freebies. At the Cult of Cherry event there were themed aprons which looked kind of cool.
From what I've heard also it might be good to get there early and then leave early too as these events sound like they get really packed to the point you can't even move in the store.
Hope that helps - have fun!