That's a great question that gets into the subtle details of urban design! While both shapes get the job done, round bins offer some distinct practical advantages in public spaces.
First, round bins are generally more space-efficient in terms of movement. They have no sharp corners, which makes them safer in high-traffic areas like sidewalks or parks where people might bump into them. This also allows them to be placed in tighter or curved spaces without creating awkward gaps.
From a usability standpoint, the circular opening is often more intuitive. People can approach and deposit trash from any angle without having to "find" the opening face. This can subtly encourage proper disposal.
Structurally, a cylinder is a strong shape that distributes pressure evenly. This makes round bins more resistant to dents and damage, especially when they're emptied by automated truck lifts that compress the contents. The circular shape also allows for more efficient compaction inside the bin itself, often leading to a slightly greater usable capacity compared to a square bin of similar footprint.
Aesthetically, many designers and city planners find the softer, continuous lines of a round bin more visually pleasing and less obtrusive in organic environments like parks or plazas. They blend into the flow of a space rather than interrupting it with hard angles.
That said, square bins have their place too—they can be better for maximizing capacity in very specific, rectangular niches or against straight walls. But for freestanding placement in the dynamic environment of a city, the humble round bin often comes out ahead in safety, durability, and user-friendliness.