There are many reasons why one might want an instructor to teach you how to windsurf. There are a coupe of reasons for which I believe you should definitely hire a windsurf instructor, especially in the initial stages of your learning. Windsurfing is an extremely technical sport in which a lot of details can be gotten wrong and internalized, hindering our progress.
As I just mentioned, learning how to windsurf on your own can be detrimental to your progress as there are a large number of things that you can do wrong. Elements such as keeping the front arm straight, avoiding the monkey stance or bending our back to pull the sail out of the water are crucial to get right from the first lesson. They are also elements which are very quickly forgotten which an instructor will detect as they occur and correct.
While learning you will get loads of well intended advice from your friends. However, they might not necessarily know which your next learning step should be. Depending on the spot you are windsurfing in you might be better off learning the beachstart before learning to plane or using the harness. I have heard of kids in Hawaii or Isla Margarita that have skipped the gybe entirely and jumped straight to the front loop (although this is probably just your standard adolescent “getting ahead of ourselves” :)).
Aside from knowing which your next step on the learning ladder should be, an instructor will be able to tell you if you are even ready for the next step. Countless times I have had students wanting to learn to waterstart when the beachstart was not solid, or wanting to learn how to carve-gybe when they weren’t planing properly.
We are talking about an extreme sport in a potentially dangerous environment. Having someone next to you while you are trying out a new maneuver is a very sensible thing to do. Many times my task for the hours lesson would be to “keep company” to students who did not feel safe at a new spot. I would accompany them with kit that was a little bit easier to handle so that if they grew tired by fighting for a waterstart in non-flat water, they could take over my gear and up-haul to sail back to the beach safely without being washed onto the rocks by the current.
Also, arriving at a new spot that is more challenging than, for example flat water, might give you a hard time. You can save yourself lots of frustration by just getting someone to check out how you are coping with the conditions like shore break or gusty winds and give you a few crucial pointers on how to save your energy or improve your control over the kit, etc.
To conclude, a windsurf instructor is a very smart decision when wanting to learn a new maneuver, arriving at a new spot or going back on the water after a longer break to file at your technique. Hiring an expert will shorten your learning curve and solidify shaky elements of your technique.