That is simple - it is about brewing and racking off cleaner beer.
One thing the conical base does is greatly reduce the contact patch/area of the trub pile with your beer during ferment. When it comes time to rack off, transfer, or bottle your beer the conical bottom also does a great job of holding the yeasty beasties down there while you take care of your business.
Sure if you want to get really technical and talk about 60-degree side walls versus 45 degrees you can ... But if you are that technical and bothered by that 15 degrees you should probably be brewing in a full on big boy conical fermenter anyway!
What is funny is that some of the “purists” out there take great offense to our 45-degree sidewalls on the conical section of our Bucket, yet we guess it doesn’t bother several of the major commercial breweries that are using Buckets in their operations every day for some of their testing and pilot batches.