iTunes asks your phone to send a few small packets of data (SHSH blobs) to apple's servers. Then, if everythings ok, they get 'signed' and sent back to iTunes. iTunes will only up/downgrade or restore a iPhone if it has these signed blobs. As far as I'm aware apple's servers will not sign blobs for phones that are restoring to a older firmware. That's why you need your blobs to downgrade so you can trick iTunes into thinking that they are signed...