The most likely reason is to capture video or a second angle while using the DSLR for stills. A small camera on the hot shoe can continuously record a wide “safety” shot or behind-the-scenes footage while the main camera is used normally.

This can be useful because compact/action cameras are small, light, and can often record continuously with less concern about clip-length limits than older DSLRs. That makes them handy as a fallback if the DSLR drops a few seconds between clips, or to provide an always-available wide view while the photographer shoots tighter still images.

In some cases, photographers also use two cameras together to capture wide and tight video angles at the same time.

So the setup is usually not about replacing the DSLR if it fails. It’s more about adding a simple, lightweight second recording device that stays aligned with the main camera and is easy to carry.