The Launcher helps you :
-change animations and the graphics of your theme
-change your icon/app drawer layout
-get customized widgets
The custom ROM helps you:
-change your kernel(for battery life,sound quality)
-allows you to overclock/underclock(this can be also done by some application ex : ROM toolbox)
-removes the carrier/manufacturer skin
-change the icon for the stock battery meter
-update the version of the OS
-Add new features
-increase the speed and stability
Hope this was helpful.
Edit : If I forgot something feel free to post it.