Do these actually work? Particularly referring to the ones which protest about some issue or the other and have people from all over the world put their names on it?
Delete them - they just put unnecessary strain on mail servers, eat bandwidth and fill your mail box with junk. Even if the cause is genuine ( which I suspect it hardly ever is ) it is still a lousy way of getting recognition.
Email is as easily faked as ... well... something really easily faked. As such, it provides no form of proof or contract in any legal system. For the same reasons, no petition based on email would ever be believed or recognised.
Do a Google search for some of the unusual-looking names higher up the chain: you'll almost definitely find that the list of signatories appears in different online 'petitions' for a variety of causes, and have been copied'n'pasted from one list to the next. Some will date back several years to boot.