How is it different if I just say "the really important ones are here" without "over"?
There is a slight difference between these two phrases. Saying "over here" indicates that an object is a little farther away from you. Just saying "here" indicates the the object is close to you or right next to you. Ex: All of my books are over here. Ex: She is sitting over there. Ex: I want you to put our dishes here. Ex: I parked my car over there.