When you play blackjack, your final hand can only be one of these possibilities--a blackjack, 21, 20, 19, 18, 17, 16 or less, or you could bust. When you bust, you know you automatically lose and when you get a blackjack hand, at worst you'll tie and at best you'll get paid a bonus of 1.5 times your bet. But what are your chances of wining when you stand on your 17s through 21s? You would hope they are good, but that's not always the case.
Let's take the example of standing on 17 in a six-deck game where the dealer stands on soft 17. Most players are happy to have a 17 and thankfully stand. However, you will lose more money than win when the dealer shows any face card but a 6. You will lose the most money when the dealer shows a 9 and the least when he shows a 4 or 5. It's only when the dealer has a 6 showing that you stand a fighting chance of winning some money in the long run.