You only need the .deb files for installations. The other are compressed source files but you need utility programs to uncompress them. You don't really need them unless you are curious to see how they were written or to fix their bugs.

But with N800 and you don't have internet access to it then you miss out a lot.

With the N800 you just browse to the place, click on the .deb file then application will be downloaded and installed.

Even if you downloaded to a PC and transfered the .deb file to your N800; you still need net access for the N800 to download the application.