Nine weights and above are for salt water and open water fishing where heavier lines are needed to make longer casts into the wind..... They are also used for the "chuck & duck" style of fishing used by Michigan steel head and salmon fishers........
A 7/8 weight rod is best suited for larger wind resistant bass bugs.......
A six weight is a good all around rod and the recommended choice for the beginner..... It is capable of casting dry flies as well as weighted nymphs.....
4/5 weights are considered dry fly rods where gentle presentations and light tippets are the order of the day.......
3 weights and under are noodle rods for those up to the challenge. These are definitely not on the novices list of needed items..........
Good Luck and Happy fishing!!!!!!!!