when watching old films on your computer, why do they put those black bars at the top and bottom of the screen

It's called "Letterboxing", and it's not just on a computer.

Letterboxing is the process of adding black bars to the top and bottom of a movie or video after shrinking the whole image to fit a smaller screen, which otherwise could not accommodate the wide resolution of the film. This is done because most movies or films are shot in a widescreen format meant for the theaters, which is wider than the format used by standard 4:3 TV and 16:9 HDTV.