Adding variety to a repetitive routine
Doing the same fixed circuit every session can get monotonous, and knowing exactly what's coming next removes some of the mental challenge of a workout. Randomizing the next exercise with a spin adds unpredictability, which some people find keeps a session more engaging.
It also works well for group fitness settings — a gym class or a friend group working out together — where a shared spin decides the next station or exercise for everyone at once, adding a bit of shared suspense to the session.