This is a common mistake made by textbooks as it is used to further teach the language but, silakan does not exactly mean please. It's meaning is closer to "go ahead" or "help yourself". You would not use it when you're asking someone a favor, or when you are requesting food at a table. "Mohon" is somewhat similar to please I suppose. (E.g. Mohon maaf means "please forgive me", or "I ask you to forgive me".)