Why do some OSX progams have an installer (mpkg) instead of drag-drop?
The drag-and-drop method originaly came from cases where the application was a stand-alone executable, so all you needed to do to install it was drag the executable from a floppy to the App folder.

Today many applications use installers that let you drag-and-drop to the app folder, while the drag action in fact launches an installer that will install multiple files in different places. So technically the drag-and-drop today may very well be an installer.

So why then use an "Next-next-next-install" installer? Because you may want to ask the user questions! Maybe you don't want to install all parts of the software? Maybe you want it installed somewhere else? Or it could simply be a question of accepting the license agreement.

You can't do that with the drag-and-drop install.