The Import PDF function allows you to import a PDF without damaging certain PDF objects that Illustrator doesn't understand, and also preserves PDF layers and their relative order.
For instance, if you just use the Open command to open a PDF in Illustrator, you will frequently see the error message "The document contains PDF objects that have been reinterpreted: An unknown shading type was encountered. An unknown imaging construct was encountered." This is a sign that certain PDF objects have been damaged, and may not display or print as expected. Additionally, all PDF layers will be lost, and all Objects will appear on the same layer.
Using the Import PDF function allows you to bring "unknown" PDF objects into Illustrator without damaging them. Additionally all original PDF layers will appear as expected as Illustrator layers.