I've seen conical fermenters in catalogs and on the web, but I've never heard why they are better than fermenting in a glass carboy.
Why would I want to use a conical instead of a carboy?
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The cone shape and valve on the bottom lets you remove the trub without transferring containers, so you can do your primary and secondary ferment in the same vessel, or bottle without worrying about the trub.
The bottom valve also makes it easy to remove your beer for bottling and such, so there's no need for siphoning. I think this is mainly advantageous when you get into batches that are a lot bigger than 5 gallons