One theory that I have heard, though I don't know how valid it is, is that the salt and/or citrus (most cases lime) put on or in Mexican beers and/or drinks was to sanitize the bottle/glass.  I can somewhat believe that since traditionally people have always said "don't drink the water" when traveling to Mexico.  Supposedly the chemicals in citrus/salt neutralize bacteria.  Again I can somewhat see the reasoning for that, but I'm not so sure that I believe it.

In my opinion the salt and lime on a drink like a margarita adds to the flavor.  I just never understood the "added flavor" adding salt and/or lime to a beer.

By the way, I confess that I do like Tecate beer (no salt or lime) though I don't usually intentionally buy it.  I view it basically as the "Mexican Budweiser".

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