Physical Preparation Before The Fight

[Guest Meerkat]

I heard that in boxing, fighters would try to gain weight after the official weight has been recorded. Is that true? If it is true, then what is the reason for doing that?

[Dust and Rust 9]

Make it harder for the opponent to bring you down? More weight makes gravity work harder? I have no idea, especially if we're talking about welter or feather weights.