Iodine flasks differ from normal conical (Erlenmeyer) flasks in three main ways:
- The ground glass joint.
- The modified stopper.
- The funnel shaped top.
Iodine itself, and iodine related molecules produced during the analysis are, to a certain extent, volatile (you can see this if you just put a bead of iodine in a sealed vial and watch the vial fill with vapour). The ground glass joint of the iodine flask prevents these vapours from escaping, improving the accuracy of your analysis.
As for the funnel, its simply there to help get your stuff into the flask in the first place, preventing loss through spillage and preventing the need for a separate glass funnel. Anything that ends up on the tapered neck is simply rinsed directly into the flask.