-If I go out and buy each card in a set individually, I will spend more than buying the complete set.
-There are some cards that are in high demand and will sell quickly if the price is right, but the majority of cards will wait for a long time before anyone is interested in buying them. (Few people want to buy
The Weight of a Legacy even at a very low price).
-Selling a set can bring in more money than selling just the high demand cards, as buyers recognize that they are getting extra stuff even though it is not highly valuable (e.g. I really only want about 10 rares from a given set with individual costs adding up to $30, but I might pay $40 for a the set because the other cards could be useful for later trades & such).