Though I haven't invested too much time verifying this, the story is that Guinness was originally founded as a Christian ministry to prevent drunkenness. Basica

Though I haven't invested too much time verifying this, the story is that Guinness was originally founded as a Christian ministry to prevent drunkenness. Basically, Arthur Guinness was seeing his beloved Ireland ravaged by alcoholism, mostly caused by abuse of gin, so he wanted something that was more difficult to get drunk on. Guinness was supposed to be heavy enough (and sufficiently low in alcohol) that your limiting factor would be feeling full, not unconsciousness.