Can someone tell why I often see fly rods, even 7 1/2 foot rods with two stripping guilds? It seems unnecessary.

It is.

Fly rods only have one stripping guide - that is the guide closest to the reel. So I assume you are talking about the use of more than one ceramic type guide.

Look and see where most of the stress is when the rod is heavily loaded. Of course, that's going to be in the lower half, most often the bottom 1/3rd. So the use of ceramic guides in that area facilitates the smoothness with which the line will pay out or be recovered under load. Very light rods may only use one, very heavy power rods may utilize three (or more). Those in the middle typically use two.

...........

Take it a step further, “fly rods” used for large blue water fish, Marlin. Typically short, 14-16wt, sort of. All the guides are the same style, typical of what you’d see on a spin or casting set up. Those rods are not really meant to cast a fly pattern, but to fight big fish.

I typically do two of the same style on 8wts and heavier. Then snakes for the rest. On the few Spey rods I have built, I used 3 of the same style then snakes for the rest.

Ah ha. I see the logic in it now. I probably still won't do it since I make mostly 3-5 wt rods, but at least I understand and appreciate why it's done.
Thank for responding.

Tom is of course correct. Technically only the first guide up from the grip on a fly rod is a stripping guide, even if the second guide is of a similar two-foot design with a ceramic or other insert. Anglers tend to call both "stripping" guides, but that is actually incorrect.

That aside, there is really no reason on a fly rod of 9 feet or less with a line rating of 6-wt or less to have a stripping guide AND a second guide of similar design, albeit a bit smaller. It's a waste and overkill.

One size 12 or 10 stripping guide on a 7.5 to 9 ft. fly rod of less than 6-wt. is sufficient, and can be followed by either a 1) tapered-down approach to wire running guide sizes, if you like those aesthetics, or 2) you can just go from the stripper to a constant small size running guide. A tapering is not required.

Personally I do quick taper as I like how it looks. On a recent 9 ft. 5 wt I went with: 1 size 10 stripper; 1 size 5 wire running guide, 1 size 3 wire running guide, all remaining wire running guides size 2. Running guides were all single foot. If they were double foot snake types I would have gone with size 1 for the remaining guides. For single foot wire running guides I often go up one size vs.double foot snakes.

For a 7.5 foot fly rod you could go with a size 10 stripper and taper down to or go to size 1 or 0/1 running guides.