Sometimes flash crashes occur and you get coins extremely cheap so a lot of people put buy orders in at very cheap prices like 10-50% of current value. They tend to jump back up in price after such big falls so it can be lucrative.

When you say ones that have zero value is that based on your own feelings that they have zero value and are over inflated? Because what has no value to you may be worth a lot more to someone else, even if its only speculators.