In days gone by, when internal plumbing hadn’t been invented, to empty the nights deposits house owners or maids would tip the bucket into the street, often from a second floor window. Even if the contents missed the sidewalk / pavement / walkway there would be splashing all over passers by. This way not the only hazard for pedestrians. Due to the fact that automobiles hadn’t yet been invented and the only form of transport really was horse power, there would often be large piles of equine doo doo. Finally horse drawn carriages and wagons would ride through puddles of water or other liquids, generating waves of wetness towards the side of the road.
“If you’re walking with your lady on the sidewalk, I still like to see a man walking street-side, to protect the lady from traffic. I grew up with that, and I hate to see something like that get lost. I still like to see that a man opens the door. I like those touches of chivalry that are fast disappearing.”
These are some of the reasons why, when walking with a lady, gentlemen would show chivalry by walking to the outside of his companion, putting himself between his lady and any splashes from above or the side as well as being the one to have to dodge any horse excrement.
Now that indoor plumbing has been invented, along with the combustable engine and automobiles, the reasons to walk on the outside have changed but chivalry hasn’t. Puddle splashing is still a danger but projectiles from above and deposits from animals are no longer. This doesn’t mean that a gentleman no longer needs to protect a lady by walking to the outside of the sidewalk. A chivalous gent should still walk to the outside of a female companion as even if the chances of a car splash situation are incredibly low, there is still a chance.