Why exactly is a root hash used in relation to hash lists?
Yes, the transfer of just the root hash can be a good reason for using a hash list. Then you can still hash the leaf hashes in a parallel fashion.

However, generally the root hash is simply needed so that the hashes aren't reordered / removed. Of course this won't protect against deliberate attack; you would have to use the root hash in a signature scheme for that.

Sending hashes together with data blocks only makes sense if you expect accidental damage. In that case you might want to use a non-secure hash or another error detecting / correcting code (such as PAR2). In general the transfer of hash lists or trees takes part separately.

In the end, a hash list is a very generic concept. How it is used depends a lot on the protocol and/or use case. Possibly that's why you don't find that much practical information on it.



Note that the Wikipedia page correctly specifies that often a root hash is used. What is probably more common is that the hashes are aggregated in a separate data structure, and that that particular data structure is signed. This data structure can include additional (meta-) information in the signature.

You can see this in e.g. XML digital signatures, the cryptographic message syntax (PKCS#7 or CMS) but also ePassports where the separate data groups containing the personal info / fingerprints are hashed separately and the result is put in a structure and signed. This way you can verify components separately, while using only a single signature.