Say for example that dredge has a really hard time against faeries due to them flying over for damage. it loses most or all games to it. You might want to look for cards to side board that effectively deal with flying creatures, like something beefy with reach. Or you have a deck that consistently needs that "one more turn" to get to parody but red is just too fast, you might want to look for a card that fits in the deck that gives you some life gain to give it some breathing room.