with the user button class select setting you can switch classes however you want, without using bodies, and without the class selector I found no way of changing classes if not dying (wich does not require bodies either)

The class select button was added because people wanted it.

Anyway. Bodies. Build a body, place it in the world, you now have 2 bodies. You can use the bodies GUI to swap between those 2 bodies.

each body has it's own set of weapons and armor, and it's own inventory, and set of Logistic request slots and trash slots.

I personally use this quite often, different bodies would have different requests, even different blueprints, and I'd pick a body, and hop on a train and do a specific job with it.
or, leave a body at a remote outpost, so when I need to go there to do something... I don't actually have to go there, I just swap to the body that is already there.

There's a lot of possible uses for multiple bodies if you experiment with them.

wow, really usefull, thanks for the help