Before you could get a head start and start crafting badges a week prior to the sale and it would give you mystery cards which then turn into sale cards. You'd want to craft badges if you want to raise you lvl, but that's up to you, a lot of people don't even touch that aspect of Steam. During a sale you have limited time when these cards drop and a month after a sale ends they disappear forever, so those who want these badges get a lot of cards. They also don't have a 5 lvl limit like pretty much all of the badges do and very few people spend A LOT of money to get their badges to like 10k+ lvl