What is a USB laser controller?
A USB controller is a Digital Analog Converter (see the >>> definition of DAC <<<) that converts computer signals into ILDA laser signals. You'll need one, if you want to design your own laser show with a show laser software on your computer and then transfer the data to your laser projector.
What is the benefit of a USB laser controller?
USB laser controllers typically don't need an additional power supply.
What is the benefit of an Ethernet laser controller?
Usually they are more easy operate. Furthermore much longer transmission distances are posssible.