Honestly, the answer depends on where you're putting it, but for a heavy concrete urban bench I never just set it on dirt or thin pavers. A bench that weighs 600–1,200 lb can crack, tilt, or become a liability if the base moves. Here's what I'd use.
First, check your site. If you're installing on an existing concrete sidewalk or plaza slab that's at least 4 inches thick and in good condition, you may not need a new foundation. Drill through the bench's mounting holes or use steel angle brackets, then anchor it with stainless or hot-dip galvanized wedge anchors or epoxy-set threaded rod. I usually use 1/2-inch or 5/8-inch anchors with at least 3–4 inches of embedment into the slab. Shim the bench level with stainless shims or non-shrink grout, then torque the anchors. This keeps it from walking or getting tipped.
If you're going into soil, grass, or a landscape bed, I recommend a reinforced concrete pad, not just gravel. A heavy concrete bench concentrated on legs can punch into soil over time. I'd do a pad at least 6 inches thick, extending 4–6 inches beyond the bench footprint on all sides. Use 4,000 psi concrete with rebar or wire mesh. Under the pad, place 4–6 inches of compacted angular gravel, like 3/4-inch crushed stone, over compacted subgrade. This drains water away from the concrete and gives a firm, level base.
In freeze-thaw climates, frost is the big issue. If you can, excavate the perimeter down to the local frost depth or use a floating slab on well-drained gravel. I'm not as strict as I would be for a building, because a bench is a small slab and can tolerate slight seasonal movement, but if you have expansive clay or wet silty soil, it's worth over-excavating and adding more gravel. A 10–12 inch compacted gravel layer under the pad usually reduces heave enough for a bench.
Anchoring is non-negotiable for urban settings. Cast anchor bolts into the wet concrete using a template that matches the bench's mounting holes. I prefer 1/2-inch stainless steel J-bolts or L-bolts embedded at least 4 inches, projecting about 1.5 inches. If the bench is already fabricated without built-in holes, use heavy-duty concrete anchors after the pad cures. Galvanized or stainless hardware only; standard steel will rust and stain the concrete.
Drainage and slope matter too. Set the top of the pad about 1/4 inch proud of surrounding grade and slope it slightly so water runs off. I aim for about 1/8 to 1/4 inch per foot away from the bench. Standing water under the bench leads to cracking, moss, and corrosion.
For very heavy benches—say 1,500 lb or more—or sites with poor soil, I'd add concrete piers below the pad or use a thickened slab with a perimeter beam. But for most standard heavy concrete benches in city parks, streetscapes, or campuses, a compacted gravel base plus a reinforced 6-inch concrete pad with embedded anchors is the sweet spot. If you're on good pavement, anchor directly.
One last thing: call before you dig. Utilities can be in the way, especially in urban areas. And check local building codes or site furniture specs; some cities have standard details for bench foundations.