You half to put something on the inside of the barrel of any firearm (or any exposed steel surface for that matter) to prevent rust.
The only case I know were this is not done is in target shooting because the extra oil in the barrel will burn during the 1st shot fired causing the 1st shot to be a teeny weeny little bit high. In a self defense or hunting application it's not enough to matter. But: a really good target shooter might lose a point because of the extra millimeter. So they don't oil their barrel after the last practice before the match.
When oiling any pistol: "less is better." I don't recommend a lot of oil unless the pistol is to be placed in storage.