What did computers without screens do?

Use the lights. Wish for a teletype Buy Don Lancaster's books (How many copies did he sell?) Not having a screen made it a little more difficult to answer the inevitable, "What is it good for?" "What can you do with it?" The answer of course was that a hobby/home computer had exactly the same usefulness as a model train going round and round. You could speculate about maybe someday making the train deliver beer, or making the home/hobby computer do "home automation" (turn lights on and off) or bookkeeping, or filing recipes, keeping track of phone numbers, etc. Most of the "uses" were better handled by magnets on the door of the refrigerator. Once you had a screen and printer, word processing became the first practical use, followed by Visicalc and Flight Simulator.