Multisport is primarily used in Triathlon, Duathlon, Swimrun, Multisport and similar. I use it a lot. Start a run, then during the run I switch to swim and back to run and so on. The watch automatically creates a multisport activity with a multisport time and distance.
You don't have to create a workout in advance to use the watch and I believe that the majority of all users don't create workouts at all. They just go for a run or ride and that's it.
It works for all activity types I've tested with except Breathworks.
The watch must have support for "MANUAL MULTISPORT ACTIVITIES". Check specs.
It is probably only the higher end watches that got it like 945, 745, Fenix, Quatix, Tactix, Enduro.