Conical base fermenters

MattP

Hi, over the years I've seen various fermenters for sale, not the normal simple tubs, but with conical bases.
Some plastic, some stainless steel, but usually much more expensive than the usual.
What are these for, and are they any more efficient than usual ?
I have some idea that they might be intended for bottom fermenting yeasts such as those used in largers ? Although if so, the conical base would give a much more restricted surface area for the yeast to work in.
Maybe they allow the sediment to be removed first, eliminating the need for syphoning and resulting in a cleaner brew ?
Does anybody have any experience of using these, know what they're for ? Are they worth the extra money ?