What can not be done by the Panels module without using the Panelizer module?
To answer your question, as in the 'title' of your question, consider reviewing the article about Creating Specialized Layouts with Panelizer. Here is part of what is written in it, and which I think are key features of Panelizer that you can't do with only the Panels module:


  Many users of Panels have taken advantage of the Panel Nodes sub-module to create customized layouts for nodes. Panelizer does that and takes things a giant step further; by integrating with Drupal 7's entity system, it can create custom layouts for any entity item.
  
  Panelizer was initially created to allow for one node to be displayed differently than any other node on the system. This allows content creators to be just about as creative with the look and feel of each unique piece of content as they would like.


Some more suggestions about tutorials that you may want to checkout:


Manage Drupal Content Layout with Panelizer.
Architecting Landing pages using Panelizer.


Regarding the Edit-In-Place (IPE, or In-Place-Editor), refer to what's mentioned also in Working with Panelizer combining panels and node displays, i.e.:


  Panelizer fully supports the IPE (In Place Editor) meaning that panelized nodes can enable the alternative directly editable panel.


The question about "Enabled Panels In-Place Editor module but cant see the edit icon" may contain other interesting information about IPE, with topics related to:


Clearing the cache.
Renderer mode.
Permissions.