The short answer is: not always, but very often they should be secured. Whether urban planter boxes need to be anchored down on a balcony depends on the wind exposure, balcony height, planter weight, railing type, local rules, and what is below the balcony. On a sheltered ground-floor terrace, a heavy planter may stay put without mechanical anchoring. On a high-rise balcony, however, even a relatively heavy box can shift in strong gusts, and lightweight plastic or fabric planters can become dangerous very quickly.
Anchoring is especially important if you live on an upper floor, if the balcony is exposed to open wind, if the planter sits on a railing or ledge, or if people or property are below. Urban planter boxes can catch wind like sails, particularly when plants are tall or dense. A planter that slides or tips can damage the balcony, injure someone, or fall over the edge. For this reason, many building managers and rental agreements require balcony planters to be secured.
You do not necessarily need to drill into the balcony. Good options include purpose-made railing brackets, adjustable clamps, heavy weighted bases, safety straps, and planter feet that lock together. Choose a method that suits the planter material and the balcony structure, and avoid drilling through waterproofing or weakening railings. Distribute weight sensibly, keep drainage clear, and check fixings regularly, especially after storms. In short, anchor urban planter boxes whenever there is any real risk of movement. If you are unsure, secure them, and check your building’s rules before installing them.