Die cracks on coins are caused by a shallow crack in the die. The crack actually creates a thin recess in the dies surface and during the strike the planchet metal fills in the recess. Die cracks are common on all U.S. Coin series and some more than others like the Lincoln Cent, and they most generally don't add any extra value to a coin.
Thanks to the internet, coin dealers and numismatic media outlets there's been a large growth in people looking for errors in their pocket change and being the next person to find the "big" discovery. With this there's been a trend in giving coins with die cracks with pet names.
A die crack can become a variety if a certain amount of coins is found with the exact die crack, since a variety must be a run of coins with the exact die crack.
However, when these are reported like the Speared Bison collectors clamor to find and buy these errors. However once the initial value spikes it eventually settles out.
So don't be in a hurry to buy the newest and pet-named mint error because it will more than likely settle out in value and be a much wiser investment with a little patience. Furthermore you might find one for face value in your pocket change or a roll from the bank.
However, die cracks are often used to identify some varieties and can be helpful in determining die stages and progression.