Or is Challenge mode exclusively for black 2?
You can transfer the modes to other games using Unova Link. Its one of three options that Unova Link provides.
As for reason to Assist Mode, not everyone can play in normal mode. There are a bunch of younger kids that play this game that don't know what grinding is, or how to train their Pokemon. So when they made a mode to make the game more fun for those that have played the games for a while, they also made a mode for those just starting.
I remember when I first started I couldn't beat Giovanni in Silph Co. Took a bunch of tries but a friend ended up telling me its cause my levels were too low and I was still using Pokemon from the beginning of the game. For those that were like I was 14 years ago, this is a good thing.