According to Brad Abrams, almost never: link
No, there is zero benefit from wrapping a BufferedStream around a
FileStream. We copied BufferedStream’s buffering logic into
FileStream about 4 years ago to encourage better default
performance… In fact, I don’t think there are any Streams in the
.NET Framework that require it, but it might be needed by custom
Stream implementations if they do not do buffering by default.