AFAIK it was discussed somewhere that PGC did not have all the logs stored (because it is really large amount of data) so it was not possible to show the difference. Probably only some checksum was stored together with some length for statistics calculation. Not sure if this changed since then.
Groundspeak does not store history of log edits either (at least not publicly) so there is really no way to get what changed unless you have stored the original email from groundspeak (with your owner log notification) and compare it manually.
With all the opt-out mail flood it is harder to manage, but it can be certainly useful if you receive edited log notification on challenge to do fast check as it is probably a change of the log type. For other caches it is mostly out of curiosity,