How do you prevent a campaign worker from substituting a box of pre-marked paper ballots for the “real one”, with both boxes having the same total count? Both boxes have a paper sticker seal with a signature on it.
Paper ballots aren’t perfect, but they’re still better than the electronics, according to computer security experts. The physical ballots can and should be observed by the public. No such observation is possible at all with the electronics.
http://news.nationalgeographic.com/news/2004/11/1101_041101_election_voting.html
The trick is to make it difficult enough that it is not worth the effort or risk.
Pure electronic machines make it too easy to cheat. Paper ballots (which can be electronically scanned) require a physical object to be manipulated. Thus leaving fingerprints and other physical evidence.