Perhaps some are thinking of creating a group planting or would like to imply a landscape effect. Graham’s slab-pots have been proving popular recently.
They have the look of a flat slab but have the advantages of a traditional pot planting. Being easier to water as the built-up edge provides protection from the drying wind. They also have a hidden foot ring allowing for proper drainage. He ensures many wire holes to allow accurate, strong positioning of the trees when building a group planting.
These can be custom built to suit an existing group shape. Other examples are shown on his Instagram feed. @dragonflybonsaipots