When a proposer proposes a block, they serialize the block, split it into equal sized chunks (we call them "parts"), and gossip those parts. The BlockID is then defined as the normal block header hash, plus a PartsHeader, which includes the number of parts and their Merkle root. The PartsHeader is a commitment to this gossip mechanism, and is signed in both the proposal and the votes as part of the BlockID. See BlockID.

Outside the real-time consensus protocol (ie. for someone who just wants the blockchain like a light client), the PartsHeader is pretty useless, so it's inclusion is a bit of an unfortunate artifact. To remove it would require some change to the mechanism in the consensus reactor for gossipping block parts. Would be really nice to have this separation, and reduce the BlockID to just the block header hash, but need to be careful doing it.

One possible idea is to chunk the block using it's existing structure. So rather than serializing into bytes and chunking arbitrarily, we'd have the header be one chunk, the transactions be a set of chunks but chunked using their existing merkle tree structure, and so on. Then the main block header hash could be still be used for secure gossipping of chunks.