The istock forums can be a bit daunting at times as they can treat you like an idiot depending on the question you ask so sometimes I prefer to go to my CN friends to ask for help or advise.
Also if you have any buyers in your CN they will often look for images in their CN first before doing a general search so that is obviously to your advantage.
Sometimes they are just people that you make friends with in the forums and you have them in your CN to make it easier to site mail them and check on their progress and uploads.