IMO, there are 2 kinds of shop. One has all you need for the local fishing, with very limited inventory for other areas, while the other has the range of inventory to cover any destination. The local shop usually has specific water nearby that they specialize in, and may also have guide services for those local waters. The better shops also have quality people, who can give good advise no matter what the type of fishing folks might be interested in doing. Nothing loses customers like BS. You can get that at the big box stores.
The full range shop may also have guides, but they'll be more of a reference to outside services at the destinations.
IMO, the best shops cover a range of equipment needs for the type anglers that Ard mentioned, and will have a full range of other items, such as tying materials.
The unfortunate thing about fly shops is none will carry everything. Most folks who own shops don't have the finances to do so. Also since fly fishing is such a niche sport, it's difficult for such shops to survive during the off seasons, unless they do things to keep their customers coming back during those off seasons.
I had a mail order/internet business for many years, and would have loved to owned a brick & mortar shop, but knew that it was not viable. I primarily tied flies & sold tying materials, but never made enough money with it to make a living. I also ended up making lures, which increased my customer base somewhat, but in my geographic area, once we got into the colder seasons, fishing stopped & hunting became the sport of interest. I didn't sell anything hunting related & most sales I had during that period was to out of state customers in warmer climates.
I also taught some fly & jig tying classes, particularly in the colder months.
You have to be very diverse, and willing to put in a lot of time to keep a shop operational. Don't think you'll spend a lot of time fishing either, unless you have good quality help to run the shop.