Computer games have been around for a few years now. Like anything else, they have a shelf life. They get superceded, or the system you're using changes from a zx81 to an Amiga to a PC.
But should you have a hankering for the good old days when you spent hours typing "Go west and pick up knife" or whatever, it's possible to get hold of PC versions of many of these old games.
The developers have recouped their costs. Or if they haven't, nobody's going to bother paying for their 20 year old code. So it's possible to find sites that will allow you to download playable games free of charge.
I'm busy playing speedball 2 right now. Management mode, so that I don't have to wreck my keyboard or strain my ageing fingers.
The best site I've found for this is called Abandonia. There are others. I use to use Home Of The Underdogs, but that used to be plagued with popups and spyware.