Television science tells us that alternate timelines exist, and that we can visit them at any time. Real-life science, party pooper that it is, tells us that even if alternate universes do exist, it would be impossible to visit or view them in any way. The concept, though, has led to some of the most interesting and entertaining episodes of television ever produced.
Each series that uses it approaches the concept of alternate universes in a different way. Shows like the Jerry O’Connell vehicle from the late ‘90s called Sliders, for example, build their plots around visiting a different alternate reality in every episode.
Television sitcoms like Friends, 3rd Rock From The Sun, Mad About You, and The Big Bang Theory, generally use it to tell what-if stories that stand aside from the regular canon of the show, playing the premise for laughs more than anything else.
Then, of course, there are the genre shows like Doctor Who, Buffy the Vampire Slayer, and others, where alternate universe episodes are most often used to make a point or teach a lesson as is traditional in the genre.
Whether or not you enjoy these episodes, their uniqueness makes them more memorable than the average television story.