I could say ditto to the many excellent comments already made. I have several rods that are labeled for multiple lines. In addition, I will often use a different line than what is labeled when a singular line is "suggested". With graphite rods, I am often inclined to go a weight heavier. While with fiberglass, I may be inclined to go lighter. I have found that with the dual rating, the lighter line is more suited to a higher line speed and greater distance. The heavier line creates increased line feel, is easier to cast on windy days, and is better for casting close. A person's individual casting style will also affect what suits him/her.

With older rods, the dual line rating often reflected the manufacturer's specific line choice suggestions. Often, the lower number would indicate the rod's suggested DT line; while the higher number would indicate the WF.

Due to the nature of fiberglass, fiberglass rods are more capable of casting a wide range of line weights than graphite. Several rod builders, notably Walton Powell, believed in the wide range of lines that their rods could reasonably be able to cast well. For example, I have a Walton Powell fiberglass rod that is labeled as a 9-10-11-12. I have tried each weight line on the rod, and while I have preferences, they all work well.