Quite often, it's a question of matching – there may be six sums of money or six objects, and if three of them are the same, that's the prize the card wins. A big advantage of scratch cards is: you know immediately whether you've won – and, if you have, what the prize is.
You can buy scratch cards anywhere you can buy an online lottery ticket. If you know how to buy a lottery ticket online, then you know how to buy a scratch card. Buying game tickets online means that anyone almost anywhere in the world can play. A bigger pool of buyers means more chances to win because there's more money in the pot.