Flash custom rom?

I think I posted this in the wrong forum earlier. I currently have a Galaxy S4 and looked at the nexus 6 if I have to buy a sealed phone vs the new galaxy. Since everything is internal when making a nandroid do I need to transfer the nandroid to a computer then wipe? If I wipe the phone then how do I flash a new ROM if the internal is wiped ? Nandroid... Ok..... Then transfer is my guess to computer..... Then wipe then flash.... Except the internal is wiped.

Everything you flash a new ROM do you have to back up and transfer everything to the computer?

With the galaxy s4 I would just wipe the phone then load off the SD card slot.