According to Rob at Needswings ( when I asked him ), the big hose actually pulls filtered air IN to the engine from the CAI to offset the vacuum that is caused by the 2 smaller hoses sucking fumes OUT.
I know this is true, because I once had the bright idea myself of capping both ends of the big hose, and the engine was NOT happy.
I had it this way on my first trip to the Dragon, and had several knowledgeable people at the lodge concur with me that my water pump was going bad.
Apparently, causing a vacuum condition inside the engine makes it squeal like a pig.