As others have said - InDesign is a page layout tool.
You can of course create a 92 page booklet with Illustrator.
But it's a pain if you need to paginate the whole thing if a page moves, or you have text threaded over 30 pages - and other things.
InDesign is a fantastic tool for creating multi-page documents that will be printed by professional printers, or for creating ePubs, very small swf files, and documents intended for the web.
It can be used to make 1 or 2 page leaflets, booklets, or many pages, posters and other things.
It's rich in Paragraph and Character styles that can be lined integrally with Grep styles, Nested Styles and other things which can all be connected with Object Styles!
The ability to add Running Heads, Text Varialbes, Page Numbers, Facing Pages, Spiro Bound books and other things with easier is far more efficient than Illustrator.
It has vast typographical controls and loads of things that Illustrator simply cannot do.
It's certainly a very complex program and takes time to learn - but once you get it - it's great.
Think about this -
I formatted a 3,200 page book using Running Head Styles that changed per page - automatically!
And not just 1 running head - each page had at most 2 running heads that needed to change dynamically
I also applied bold, italics, bold italics, to certain words throughout - all with the flick of a switch.
What used to take me 6 months to do with Quark took me 2 weeks with InDesign.