You certainly don't need to. It allows you to put more apps on your NIT. It allows you to do a better (a real) backup or to clone a system and apps to go in another NIT. It allows you to have alternative system-and-app configurations. Some users like to install KDE (which apparently takes a lot more room than I allocated in the guide). It might make your N800 trivially faster. It's pretty esoteric, really, and there is no need to do it at all.