Hello Donco15,
The backup USB can serve a few different purposes:
1. It’s useful in situations where internet is slow or security is a bit tighter. The software still needs to activate online, but the content necessary to install is already there.
2. It’s a backup medium for the program installer, core catalog content, and other resources. Some people find it handy so they don’t have to track them down on the website if they need it down the road.
I hope this helps!