Two big reasons for Layout Mode:

1.  You have a module that is only visible to Unauthenticated users but can only be modified by Authenticated users.  You'd need Layout mode to be able to see that module's menu icon so that you can change the settings (making it temporarily visible to all users so that you can edit its contents or to change other settings).

2.  You have a container whose menu icon is on the right-hand edge.  The contents of module exceed the visible width.  It can be tricky to access the module's settings unless you're in Layout Mode.

The change of making Design Mode/Layout Mode a separate view (rather than an On/Off toggle) was a vast improvement (many people would get into Design Mode by mistake and struggle with it).  Many people will never have a reason for switching to Layout Mode....but it's a lifesaver when you actualy DO need it.