Short answer: false bottoms in urban planter boxes can be genuinely useful, but they are not automatically a drainage solution. A false bottom is simply a raised platform inside the container. It creates a gap between the soil and the bottom of the box. Whether that gap helps or hurts depends on how the box handles water.
In a good design, the false bottom does three jobs. First, it keeps the main soil mass out of standing water. Second, it creates a reservoir that can store some water for hot, windy balconies and rooftops. Third, it leaves an air gap or drainage layer so roots near the bottom are not permanently soggy. Many self-watering planters use this principle: water sits below the platform, a wick or felt strip pulls moisture upward, and an overflow hole prevents the reservoir from rising into the soil.
For drainage alone, however, a false bottom is often unnecessary. A normal container with properly sized drainage holes, a free-draining potting mix, and a saucer or tray can work very well. If the false bottom has no overflow, no wicking material, or no way for excess water to escape, it becomes a gimmick. Water collects underneath, the soil above stays saturated, and roots suffocate or rot. A sealed false bottom is worse than no false bottom.
The key detail is the overflow. In a useful urban planter, the overflow should be set slightly above the false bottom so a shallow reservoir remains, but any extra water drains away. If the overflow is at the very bottom, the reservoir empties completely and the box behaves more like a standard drained pot. If the overflow is too high or missing, you get a perched water table and root problems.
False bottoms can also be useful in rooftop and balcony gardens because they reduce the amount of heavy soil needed and can make large planters easier to move or assemble. But they do not eliminate weight: water weighs about 1 kg per liter, so a full reservoir adds load. They also need maintenance. Drainage holes clog, wicks degrade, and fabric barriers can become root-bound or matted.
So are they useful or a gimmick? They are useful when they are part of a complete water-management system: overflow drainage, wicking, air gap, and a suitable growing mix. They are a gimmick when they are just a plastic grid sold as a magic drainage layer. Before buying or building one, check three things: Where does excess water go? Can the reservoir drain? And can roots stay out of standing water? If you can answer those clearly, a false bottom is a practical tool, not a trick.