One might be driven to prop a screen door open with a cinder block if one had multiple small children coming in and out of that door, slamming it fifty thousand times in rapid succession. Or perhaps if it wouldn't stay closed on it's own and kept smacking the side of the house?
I've got three grown, all two years apart. There was a cinder block propping my screen door open most of the time from about 1984 to 2002.
That eliminated a lot of the "WILL YOU PLEASE STOP SLAMMING THAT G*D D*MN DOOR!!!!!"
This takes me right ack to the vivid memory I have of first propping that door open like that. Husband came home, looked at that block, and said "where'd you find THAT?"