It's like bufio.Reader that reads as much bytes as possible in one read call to reduce the number of Read calls to underlying net.Conn which results in system calls.
But it returns its internal buffer to the caller to reduce memory copies. Most of the times, the caller unmarshal the slice to something else (like protobuf or just make an int out of 4 bytes).
Internally it uses sync.Pool to further reduce gc.
reader := bufreader.NewBufReader(conn, 1024) // use the reader to read the content in conn
length, err := reader.ReadFull(2) // read 2 bytes for length
data, err := reader.ReadFull(int(binary.LittleEndian.Uint16(length))) // read data