Bump again!

Can anyone give me some insight about what's going on with the fastening devices on the rear wheels of many kids' bikes?

Some of them are simple and just have a 15mm axle nut.

Others have a 3-piece setup, and those are the ones that confuse me:
- 15mm axle nut on the outside
- Some sort of "plate" just inside
- 17mm nut further inside of the plate

It looks like this:



What's the purpose of the plate piece? It's not acting as some sort of washer, is it? I've seen kids' bikes where it looked like the plate fit over the training wheels some way... but what exactly is the plate doing?

Also, why 3 pieces instead of just a single axle nut? In those cases, is it really the 17 that's securing the wheel in place? And that way you can add/remove training wheels by popping off the 15 on the outside without worrying about screwing up the alignment of the wheel?

I want to understand everything about how they are set up, all the moreso because occasionally a bike will be missing one of the above-mentioned parts (like have a plate on one side but not the other).