We all have a main menu, which always shows. But on a busy site with lots of pages - when you are creating content, what you don't want to have to worry about is creating more menu links.
There is no need with ArtCats. It will generate a dynamic menu that offers the best menu options for you based on where you are. At top level it will show you a list of sections, when you select a section it will display the categories in that section. Select a category and you see the articles in the category (and it does it all automatically).
There are filters to work for certain sections and categories too, if you don't want it on every page.
My only reservation is that when I click on the article itself - the module renders nothing. Just a blank. I would prefer that it still showed the category listing for that category. I had a look and could work out how to hack it to get it to do it :(. I hope that this is available soon and then I'd recommend this mod for every 1.5 site.