[Go] why the decode method use reader as input params

[someview]

I have some question about the generator for go. Why use reader as decode input params insteadof []byte or bytes.buffer. For most secenes,
we need start routine for a receive message:this means we need get a business msg buffer, then parse it in other routine. so,why not use []byte or bytes.buffer as input param. Now:

buf := make([]byte,512)
n,err:= conn.Read()
go func() {
   reader := bytes.NewReader(buf[:n])
   xxx.Decode(reader,) // decode buf  to sbe object
}

Actually:

   xxx.Decode([]byte) // decode buf  to sbe object
   xxx.Decode(bytes.buffer) // this is ok