Nobody likes the robber on one of their tiles, and often just the threat of a face-down Knight card can serve as a deterrent – players may be worried that you will move the knight to one of their tiles in revenge.
The most common type of development card you will encounter is the Knight. Knights are useful for two main reasons – keeping the robber off of your tiles, and getting the largest army. Nobody likes the robber on one of their tiles, and often just the threat of a face-down Knight card can serve as a deterrent – players may be worried that you will move the knight to one of their tiles in revenge. For this reason, some players will lie and pretend that their development card is a knight, even when it isn’t. I would not encourage this strategy, as it could lead to the other players discovering that you are lying, and no longer trusting you.