It’s not added to an existing piece (or play) but written with the kind, length and ibtntensity of laughter. There are videos with the laugh tracks removed where you can see the small breaks of the cast waiting for the laugh track to stop. Not just is there a timing difference with and without the laugh track, no, jokes themselves work differently. Meaning: without a laugh track you might not be able to tell joke from serious because the guide is missing allowing to laugh (e.g. about the misery, clumsiness, idiocy, etc of a character). Sitcoms without laugh tracks expect compassion to do its magic and is probably much more closely tied to cultural context of the work. Nobody has to tell you what’s funny with Malcolm in the Middle or The Office, you know, because you once experienced (or observed) similar.