Adjunct is grafted onto the side of the regular academic ladder, which goes assistant —> associate -> full professor, and then perhaps a named chair or professsorship (e.g. Lucasian Professor of Mathematics)

The idea behind adjuncts was originally that you might hire someone on a temporary basis, either because of an unexpectedly large class or to offer a class on a special topic: a famous politician might offer a seminar about their time in office or an industrial researcher might offer something related to their work.

However, it’s become increasingly common to hire adjuncts to teach intro classes because they’re cheaper than professors (usually a flat fee per class and minimal benefits) and don’t involve much commitment (usually hired per semester), both of which have obvious short-term benefits to the administration.