FEZ Cobra II Extender

What is the purpose of the matrix of holes in the FEZ Cobra II Extender board?

You can solder any components there and connect them as you need.
So it’s for rapid prototyping.

Can I mount Gadgeteer modules such as the RS-232 or RS-485 module using holes and stand-off screws?

I just had a look at it in the catalog.
This one is new to me.
So please forget what I wrote above.
These holes are not for electronic components,.
They really look like mounting holes for Gadgeteer modules.
The Drawings on the resources tab also say the holes are Gadgeteer compatible.

Yes, they are to mount gadgeteer modules, but not many. I think the size is based on a standard plastic enclosure. And a tip: with a stand-off the battery can fit under a module.