Most laugh tracks for TV were recorded in the 1950s, which means many of the people you hear laughing are dead!
In the early days of TV, comedies were shot in front of live studio audiences and were performed as short plays. I the 1940s and 50s that began to change and closed sets were used. This gave the director the ability for multiple takes and different shots and angles.
However, it inhibited the instant feedback of the live audience. TV executives feared that without the prompting of audience laughter, the home viewer would cease to react, as well. Thus, the laugh track or canned laughter was created to prompt the viewers at home.
With time, as we all know, the uproarious laughter was greatly abused. In the 1970s, the studio audience came back with the multi-camera format. The real laughter and reactions helped writers make better jokes and sitcoms became more popular.
Although annoying, at its height, the laugh track was quite sophisticated. It was a special blend and mix of all types of laughter to create the illusion of a real audience.