The pre-install scripts may, for example, move some files/folders aside. (They can do other things, but I'm going to focus on gleaning from the scripts what gets installed where.) For example, the pre-install script for QuickTime Player checks to see if you already have version 7 of QuickTime Player at /Applications/"QuickTime Player.app". If so, it moves it to /Applications/Utilities/"QuickTime Player 7.app" so it won't be overwritten during the "Moving Items Into Place" phase. Similarly, the newer "iWork" applications rescue older versions into /Applications/"iWork '09".
The post-install scripts can do things that don't fit under the umbrella of "Moving Items". For example, this is where ACLs would be added. (Items in an archive cannot have ACLs.) Ditto with suid/sgid (set user/group id); items in an archive will have their suid/sgid bits stripped during unarchiving, so if they're needed they need to be added back in a post-install script. A post-install script can also add items to the Dock and/or the Finder sidebar and/or Login Items.
Incompatible pre-existing items can be removed during post-install. (For example, Lion's installer moves 32-bit kexts into /"Incompatible Software".)
All of this pre-install/post-install moving of files around definitely falls under the umbrella of "what gets installed where", but you won't find the items being moved inside the package.
