An introducing broker partners with a futures commission merchant (FCM) to execute customer trades. There are various introducing brokers who use Interactive Brokers as their FCM. Why would I use an introducing broker instead of an FCM like Interactive Brokers (where I have an account) directly? Because the introducing broker may provide an interface that is better in some way than that of the FCM? Or because the introducing broker may partner with multiple FCMs, giving you access to more markets?