That's an excellent question for any urban gardener. The short answer is: while not always mandatory, using a proper liner or fabric inside your planter boxes is highly recommended for plant health and planter longevity.
The primary purpose of a liner is to create a barrier between the moist soil and the planter's interior walls. For wooden planters, especially those made of cedar or pine, a liner is crucial to prevent rot and extend the structure's life. For metal planters, a liner can protect against corrosion. A fabric or mesh layer at the bottom, often called a geotextile fabric, is vital for drainage. It allows excess water to escape while preventing soil from washing out through the drainage holes, which keeps the roots healthy and avoids waterlogging.
You don't necessarily need "special" materials. Common and effective options include landscape fabric, burlap, or even a simple plastic sheet with drainage holes punched in it. The key is to ensure your setup promotes good drainage and aeration. Simply line the bottom and sides, secure it in place, and cut openings for any pre-drilled drainage holes. This simple step protects your investment and creates a better environment for your plants to thrive in the city.