When a face-up monster you control is flipped face-down, if you control no face-up monsters: You can Special Summon this card from your hand in Defense Position
Cards like Red Card and Mars generally are used to cut down the number of cards in the opponent's hand hence reducing their option, but if you're unlucky, those cards may instead fetch exactly the card your opponent needs to win the game
Neither category does much to directly aid the other, so they are usually played to assist a Deck's strategy in a non-Nemeses Deck
You are always going to find something great in every pack
It is a weaker fit for a buyer who needs exact cards, guaranteed uniqueness, guaranteed grades, or immediate turnover at a fixed margin
Other stores, they would just sell as many as they can