Those who have been fly fishing for a several years probably have several rods for different purposes. Each rod is designed for a purpose - to cast a particular weight line. Why? Let's start with how rods are defined. The size number of the fly rod is directly tied to the size or number of the fly line intended to be cast. The numbers and sizes work like shoe sizes.
A 3-weight rod will ideally cast a 3-weight line. An 8-weight rod will ideally cast an 8-weight line. The bigger the number, the larger the rod and the heavier the line the rod will cast.
Keep in mind, you do not cast the fly rod. You cast the fly line. A fly rod is simply a lever or extension of your arm. It is possible to cast a fly line without any rod at all, but not for very long. Fly rods are actually machines or tools that allow you to cast the fly line very comfortably even when casting big rods on saltwater for extended periods of time.
The fly goes along for the ride. When the fly is too heavy for the line, the line sags and the cast doesn't make it. The size of the fly you want to use determines the size of the line that will properly cast it.
Does that mean you need a different rod for every size fly you use? No. There are differences of opinion of course, but if you want a rod primarily for fishing streams and ponds, a 5-weight rod and line will do. If you are a bit more sophisticated and are going to fish gin-clear spring creeks with tiny, dry flies, a 3-weight rod and line would produce a more delicate presentation.
Fishing for salt water fish off of ocean beaches requires a much larger fly, longer casts and a rod that will handle a minimum of 7- or 8-weight lines.
But this is only part of the story. Each rod manufacturer has their own idea of what makes a good fly rod. If you cast a dozen or more fly rods, each the same 'weight' and each with identical fly lines, you will make a huge discovery. Next time, we will talk about the action of fly rods — or, "Are all fly rods of the same weight the same?"