This has been discussed many times already on the forum, and the question has been added following user's requests. Previously, it always downloaded the latest version of the AFCADs without asking first. Now, it asks first.
The reasons for this is that, when it was downloading them automatically, it created a BACKUP of the existing AFCAD so, in case you like to modify the AFCAD, you won't lose your modifications.
Some users that DO like to modify the AFCAD, found the procedure to restore from the backup (which contained their own modifications) to be a bit annoying, so we added the question to download or not.
So, basically:
1) If you like to modify the AFCAD, you might want to reply NO, although I wouldn't suggest doing this, and would rather say to reply YES and check the backup then, because replying NO might result in not getting the latest file, and this might be useful even for AFCAD-tweakers.
2) If you don't modify the AFCAD, you should always reply YES.