Why laptop is necessary for Star diagnpsis C3/SDconnect C4?

Recently, many Mercedes owners have been looking on mbstarshop for Star diagnostic units but most of them really don't know why the laptop is necessary but see most come with a hard drive. Does the software need to be on it's own hard drive or can it be put on their existing drive if they have room? Here, we customer service would like to help will-be buyers out.

Quoted from users on specialized MB forums:
---Need to connect directly into a RS232 port without using an interface - this is why I went for the complete system with laptop.