The primary benefit of Desktop is being able to use a MagneSafe, PAX, and Check Scanner reader for transactions. Desktop does allow you to manually enter transactions as well. You get the convenience of a terminal combined with the power and security of the Gateway!
You can install the program on as many computers as you would like. Just download the zip file, unzip it, and install it like any other program. Connect your hardware, launch the program, and start processing. It's easy!