[CORS] Understanding the definition of simple headers

The latest CORS spec defines the simple headers as Accept, Accept-Language
and Content-Language. However the spec doesn't provide any insight into why
these particular headers are special. What is the motivation for defining
these as simple headers? My initial assumption was that a preflight was
required for any cross-origin request that couldn't be done before the CORS
spec existed. But its not clear to me how an author could set these simple
headers on cross-origin requests before CORS.

Thanks,
Monsur