I am by no means an expert but I know some things.

I share a lot of your questions too.

5) What are GTPlayground "Bindings" and in which case does the "Refresh" make sense?

The bindings show which variables have been defined in the Playground's workspace and which object are they linked to.

Variables are created when you execute code such as:

number := 24.

Have you executed code like below:

| text |

text := 'Hello ninjas'.

This wouldn't generate a binding.

To know what's binded is important so you know if you are leaking objects or if you have unwittingly declared a variable and you script is working oddly.