Furniture sits on the floor and can be dragged out on a wet afternoon. Built-in joinery is framed with the room, shares its structure and often sits over its services, and that is permanent. It buys a piece fitting the space exactly, with no gap to lose things down the back of, and it turns every panel into a question about what will never be reachable again.
A bench with a lift-up lid is a damp box unless it is designed not to be. The frame is straightforward — built like a short stud wall lying down, because it carries a seated adult and the load has to reach the floor rather than travel through the lid. The interior is the difficult part, since what goes into a bathroom bench is wet: towels, a bath mat, swimming kit. It needs air moving through it, which means a slotted or grilled front rail and a gap behind, an interior lining that tolerates damp, and a lid stay so the lid cannot drop on fingers. A sealed, unventilated box in a bathroom grows mould, quickly and predictably.
Behind a knee wall in a sloped-ceiling bathroom sits the cheapest storage in the house, provided one thing is respected. That triangular space is normally outside the insulated envelope, so the back of any cupboard put there is an exterior condition. Insulation and air barrier have to continue behind and above the cupboard, and the door has to be treated as part of that barrier — insulated and weatherstripped rather than hung as a cabinet door. Skip it and the result is a cold void full of bathroom air, with condensation forming on whatever is stored in it.
Two smaller cases are worth naming. Under a window, the sill sets the top and the joinery must not box in a heat register, while condensation running off glass lands on that top surface and has to be shed rather than absorbed. At the end of a bath, the argument for a panel is access rather than storage, and it therefore has to be removable.