Here's the history lesson, the great Victorian docks were home to thousands of dock workers who would toil away on long shifts. Hard work and long hours. At the end of a shift they needed liquid and food, there was the breweries supplying this dark thirst quenching beer. This meant they could get food and drink at the same time. Beer was food as well as drink!!
Now we know better, you need a bag of crisps of if you want some food, but the beer is still called porter after the porters stevedores and dockworkers who drank it by the gallon!