why recommend \relative to take a "c"?

[Mark Polesky]

In NR 1.1.1 "Writing pitches" -- Relative octave entry,
"it is recommended that [startpitch] be an octave of c."

Why?  Most of the time it won't even be the "startpitch".
Perhaps "referencepitch" is more accurate, but what's wrong
with "\relative fis"?  Doing "\relative c" when the first
note is F-sharp is weird to me.  It's like when the
orchestra tunes to A440 and then plays a symphony in E-flat.

Is there some advantage that I'm overlooking?

- Mark