With ceramic pots, I imagine it's easier to get sloped sides cleanly out of the mold when manufacturing. This may be true of some plastic manufacturing, too.
When transplanting, it's easier to get the plant out, complete with all the soil, if the sides are sloped. For seedlings, the reduction in root hair damage may also be significant. Once it's free from the bottom, there's no contact with the sides, if you pull out straight. With a cylinder, the soil is scraping against the sides all the way up. If the sides are really slippery, like most plastics, this doesn't matter too much.
This makes the most difference when transplanting seedlings, and plants where the pot is still larger than the plant needs. For seedlings, the reduction in root hair damage may also be significant. At the other extreme, larger root-bound plants will usually hold the soil together regardless of pot shape.
As for styrofoam seedling trays which are often cylindrical - the shape is certainly a problem for me. The higher friction of them makes getting seedling out of them cleanly a lot harder than with other materials. The shape compounds the problem. Tried them, didn't like them. They're probably fine for growers for whom the seedlings are always root-bound before transplanting.