Reading the instructions would have told you that you need this and it is an important one. Actually from the USA you do not need 1 but 2 police certificates. One from the FBI and one from the state in which you resided.
Many people claim that they are not mandatory. The instructions though say that FBI one is mandatory and the state one is something you might or might not need and you should consult your visa office.
Another note is that many people argue that you shouldn't send anything after the initial application unless the VO asks you for it, and others argue that as soon as you get something you should send it.
Bottom line is that the FBI certificate might take upt to 5 months, so you NEED to start as soon as possible
Thanks. Just read the instruction again and found that you need to provide the certificate as long as u live in a place for consecutive 6 months. We will do it ASAP. We just wonder, will our application package be returned? (we just submitted our application last week). Our NOC is 2121.