The Navigator in Impress is completely wasted, as it serves no navigational
purpose. (We can already go to the other sides using other mechanisms, so the
plain list of slides is no great thing).

But it can be used for some special purposes. 

For example, just like the "tables" or "headings" trees in Writer, the Navigator
in Impress can show the following-

1. "custom shows" tree 
(this tree has all custom shows as its nodes. Each "custom show" node shows
slides contained in that custom show. Double-click anywhere to jump there.

2. "Slides" tree. 

the hierarchy is- Slides > Groups of objects > Objects.

Objects that are not grouped would be shown directly under the slide's name.
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With this, the navigator would be worth using (as in Writer, which has a
beautiful implemented Navigator)