It seems sodium is antibacterial and prevents botulism. Removing water by the heating process as you stated DOC, also reduces bacterial growth. It was a two pronged approach to meat preservation that they discovered generations ago.
As an added bonus, if you use the right wood smoke it tastes good too! On the downside, sodium nitrate is a known carcinogen and sodium causes cardiovascular problems.
http://recipes.howstuffworks.com/question233.htm
http://www.extension.umn.edu/distribution/nutrition/DJ0974.html
http://www.askthemeatman.com/how_and_why_to_use_sodium_nitrite.htm
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