Cats
beat a good alley for hiding stuff that you were scared to bring in the house. "Dirty magazines" shared among friends could be stuffed in some old newspapers and stored in a box at the end of a more deserted strip. While they still might eventually get discovered by a building's "super" (the superintendent or janitor), that was way better than having the stuff discovered by mom
Of course the super might chase you away from the alley, no matter what you doing. Protecting a window from being broken by a ball was a lot more important than encouraging our youthful sense of fun and adventure. "You kids better get out from back there and go play in the street before I catch you," would effectively send us scurrying towards the unobstructed daylight and the protection of the public space.
The range of alleyway activities was also dependent upon the corridor's accessibility. In some neighborhoods, alleys were enclosed by the fences and gates that delineated property lines. In others they formed a relatively unobstructed set of paths along the inside of a block, enabling short cuts by foot (or in some cases by bike) throughout the neighborhood. You could have a pretty good race, with the big kids riding their bikes around the block, the littler ones running through it.
There were practical household functions that made use of these narrow corridors, the most common being moms' hanging out the wet clothes to dry. I've heard of kids stringing lines between adjoining buildings to send things back and forth, but my friend whose family lived up in Washington Heights had us all beat. His apartment was in a building on 141st street between Broadway and Riverside Drive. His cousins lived on the same block facing 140th. One day his Dad went out and strung a telephone wire to create a working closed phone line between the two apartments. Whenever one of the aunts wanted to speak to her sister, she'd give a single ring through Ma Bell, then the other would pick up on their private line