Walk into any flower shop or look at the cooler in a supermarket floral department, and you will see stems standing in tall buckets with tops that flare out wider than the base. The shape looks decorative, like someone designed a vessel that would show off flowers. That is not what happened. The bucket is solving problems that have everything to do with keeping stems alive and nothing to do with display.
A florist's bucket is built around the mechanics of hydration and the realities of a workday spent pulling stems in and out. The flared top, the height, and even the degree of taper are all responses to specific problems that show up when you are keeping hundreds of stems fresh in a small space.