Short answer: it depends on the setting and how the bench will be used. Street benches in public areas should almost always be anchored with concrete. A bench can sit directly on grass in a private yard, but that does not mean it will stay level, safe, or easy to maintain over time.
When a bench sits on grass without anchors, the legs slowly sink into the soil, especially after rain or watering. This creates a wobbly, uneven seat and can damage the lawn around the legs. Grass also grows up around the frame, making trimming difficult, and an unanchored bench is easier to move, tip, or steal. Lightweight benches are the worst offenders, but even heavy benches can settle unevenly on soft ground.
Concrete anchors eliminate most of these problems. The typical approach is to pour a small concrete pad or footing under each leg, then bolt the bench to the cured concrete. Some installers use threaded rods or anchor brackets set in concrete for a cleaner look. This method keeps the bench stable, level, and secure, which is why it is standard for street furniture, park benches, and commercial seating. Anchoring is especially important on slopes, sandy soil, or anywhere the bench will be used by children, older adults, or large numbers of people.
That said, not every bench needs a full concrete anchor. If you have a heavy bench with wide, flat legs and you are placing it on firm, well-drained grass, it can work without anchors for a while. You can improve stability by setting each leg on a paver or a compacted gravel pad. There are also ground spikes and helical anchors that screw into the soil and provide extra hold without permanent concrete. These are good middle-ground options if you want the bench to be semi-permanent or relocatable.
The practical rule is simple: for streets, parks, commercial spaces, or any public area, anchor the bench with concrete. It protects users and extends the life of the bench. For a private lawn where flexibility matters more than permanence, you can let the bench sit on grass, but use a level base and check it regularly for sinking or rocking. If it starts to move, add pavers or switch to anchors before it becomes a safety hazard.