Imagine a rectangular frame with hinges in all four corners. Fit it with two diagonal wires ("X"). When both wires are tight the frame is stiff and keeps its shape, remove them and the frame will fold on the hinges.

The tramp in a cat is very similar to the wire diagonals in a rectangular frame, it increases the stifness of the rectangle formed by hulls and beams.

A tight tramp also pre-tensions the pair of hulls and the pair of beams towards each other, helping keep each pair together under stress.