What you just said wouldnt work the way you think it would. The visitor to the site would still be downloading the whole page again, including the navigation bar or what ever.
Using frames is like having two seperate windows open. In each seperate window you'd have a seperate pages loading. If you clicked a link in one frame (and it was set to load in another) the second frame would reload with the new information and the first frame would stay the same (no reloading). Because of this, using frames can cut-down on download times although in this day and age we all should beable to optmise navigational images to a minimum.
And, just for the record, I prefer tables.
Spu
LOVE TABLES - HATE FRAMES