Another reason could be to ensure that you have at most one copy of the application ever, since you can force it to install stuff always at the same location.
On an unrelated product we learned that users ended up with many different copies of the app scattered throughout the system, if they were allowed to use the traditional bundle + DMG distribution method.
Spotlight would then helpfully pick one random copy, with obvious consequences wrt. project file versioning.
That is despite the DMG having the usual symlink to /Applications for a drag-and-drop installation.
On an unrelated product we learned that users ended up with many different copies of the app scattered throughout the system, if they were allowed to use the traditional bundle + DMG distribution method. Spotlight would then helpfully pick one random copy, with obvious consequences wrt. project file versioning. That is despite the DMG having the usual symlink to /Applications for a drag-and-drop installation.