The Gallery Cache is basically a cache of all your media files - if you don't move them around / delete them very often, it won't make too much of a difference, b/c that is partly how it generates thumbnails. However, if you tend to move things around a lot, then clearing the cache can help remove all those cached files in the database that are no longer in the original locations.
As for tabular form, I haven't really found one. I know that some task managers can do that, but you can easily see which apps are using the most space when you go to manage apps - in the all tab, hit the menu softkey on the phone and you can toggle the sort order between alphabetical and size.