I'm confused. Doesnt the internal memory get wiped when you go to install a new ROM etc? If that happens wouldn't that mean the nandroid would get deleted as well?
No it doesn't. Only certain partitions get wiped (system, data, cache, etc.). This is only a small part of the internal memory. The rest of the memory is seen as an SD card to the phone, and doesn't get wiped unless you do a master clear in Odin (or maybe a data reset in settings will do it, but I've never tried). Installing a ROM won't touch the SD part of the internal memory.