Where they earn their keep
Either side of a fireplace. Under a stair. A dining banquette that seats more people in less floor area than chairs. A home office wall that replaces four pieces of furniture. And in Florida specifically, a drop zone at the door from the garage, which is where the whole household actually enters.
Where custom built ins in Orlando pay for themselves
In the rooms nobody has managed to furnish. An alcove too shallow for a bookcase, a landing too narrow for anything freestanding, the wall beside a fireplace that always ends up with a lamp on the floor. Custom built ins turn those into storage that reads as architecture, and because they are fixed they do not have to leave a walking gap the way furniture does. The gain in usable floor is often larger than the piece itself.
What custom built ins need from the room first
Power and a level floor. A media wall wants circuits, data and ventilation designed in before the carcasses are made, not chased in afterwards. And because almost nothing in an older Florida house is plumb or level, the scribe allowance at the wall and ceiling is what separates a fitted piece from a wardrobe pushed against a wall.
Built on site or in a shop
Shop-built carcasses with site-scribed trim is usually the right combination: better finish quality, and it still fits a wall that is not plumb. Fully site-built makes sense for something large and simple; fully shop-built only where the opening is genuinely square, which in an older house it rarely is.