The Assist function is for BMW Emergency Services, e.g. if you breakdown and your car needs towing.
I never had the Assist menu when I bought the car, but when I upgraded the firmware with my V26 disc, it appeared on the menu. That was on my MK3 nav computer. It looked nice on the menu but didn't really do anything on my car. If you selected it, a message would come up saying not available.
When I swapped to MK4 nav it disappeared from the menu. I upgraded it to V26, then V28 but it didn't appear again.
What nav computer are you running? You are welcome to borrow my firmware upgrade discs if you wish