All hives need a good roof…
A good roof shelters the box from the Sun and rain. If a colony gets too hot it can die. If water runs down the side of the box it can sit in any gaps or joins and over time rot the wood, or even flood the colony.
A well made box will have no gaps and nicely fitting joins but water or moisture can get in to anything over time. Wood can and does shrink and expand and this can cause gaps for moisture to enter.