It's how ComiXology began. A free service to fans to let them create pull lists of comics to give their retailer, so as not to miss out on that week's most in-demand titles. And, of course, creating a database to sell digital comic books directly to, at a later date.
But the pull list has survived as a feature. People still use it. A lot. But, along with In-App purchase it will be no longer available on the iOS app.
Customers will still have access on the Pull List website. But that's all.