For 10GBASE-T / IEEE 802.3ae the Physical Coding Sublayer is extended with a low-density parity-check (LDPC) code.
IEEE 802.3™-2012 18.104.22.168 Physical Coding Sublayer (PCS):
The 3259 bits are divided into 3 × 512 bits and 1723 bits. The 3 × 512 bits [...] remain uncoded. The 1723 bits are encoded by a systematic LDPC(1723,2048) encoder [...]
Using a Forward Error Correction (FEC) code increases the reliability, because transmission errors could be corrected by the receiver. Two drawbacks of FEC are the increased message size and additional encoding and decoding implementations.
However, the bit rate of LDPC codes increases with the message length and the encoding and decoding operations scale (almost) linearly1 to the message length.
What is the benefit of sending some data uncoded, especially as the encoding and decoding of LDPC is already used for the remaining data?
1the encoding of ldpc scales (depending of the implementation) between O(N^2) and O(N+g^2)