cheaper commissions and more hand-holding supposedly. If you have issues, a good introducing broker would talk to IB on your behalf to try to resolve the issue instead of you having to deal with IB directly (trust me, it's not always a pleasant experience)
It depends on the arrangement between the Introducing Broker and the underlying broker. Some Introducing Broker even actively manages their clients' account(s). Some can get access to your account information including your trades in order for them to calculate the commission accurately. For the specifics, it's best that you ask the Introducing Broker itself to find out exactly what they can get access to.
