Additives can be mixed with your paint to change it's properties; however, it may have an unintended change as well. They do not contain binder, so they will dilute your paint. Mediums on the other hand do have a binder, so they can be used for nearly any changes you want to make to your paint. You can use them to add a gloss, matte, or semi gloss finish. They can create texture or smoothness and lengthen dry time without causing streakiness lowering the quality of your materials or finished project.