Rod designers assign a line wt rating to a rod based on the line that they feel will work best over the range of conditions that they feel the rod will be used most often. This is very subjective. Most lines are rated based on the weight of the first thirty feet. Extend more line out and you are casting a heavier weight. Cast less line and you are casting less weight and may find it more difficult to load the rod for effortless casting. Many rods are designed to optimally load with 35 to 40 ft of the rated line extended.

The line carries the fly to the target. The larger and or more wind resistant the fly the greater the mass of flyline requiered to effectively carry the fly.

If you are going to be making shorter casts and/or casting larger flies it may be helpful to go up one or two line sizes.

When fishing for bass my casts are usually less than 45 feet including the length of the rod and leader. So I usually upline by one or two line sizes. I regularly use an 8wt line on my 6wt rods, 10wt lines on my 8wt rods and so on.