Who is Jabez Wilson?

Jabez Wilson is one of Sir Arthur Conan Doyle's most interesting and memorable characters. He is comical and serious, dumb and sharp, active and lethargic, well dressed and slovenly. His involvement in the actual crime is only incidental, yet he and his shop and his cellar, and his assistant, and his greed, and his defensive pride are all-important to the outcome of the story. This is probably why the author directs such close attention to Wilson's appearance. Both Holmes and Watson look him over and draw conclusions about him. For example, Watson says:

I did not gain very much, however, by my inspection. Our visitor bore every mark of being an average commonplace British tradesman, obese, pompous, and slow. He wore rather baggy grey shepherd's check trousers, a not over-clean black frock-coat, unbuttoned in the front, and a drab waistcoat with a heavy brassy Albert chain, and a square pierced bit of metal dangling down as an ornament. A frayed top-hat and a faded brown overcoat with a wrinkled velvet collar lay upon a chair beside him. Altogether, look as I would, there was nothing remarkable about the man save his blazing red head, and the expression of extreme chagrin and discontent upon his features.

Jabez Wilson's main contribution to the story is that his character makes the preposterous concept of a "League of Red-Headed Men" seem believable. Doyle had to sell that concept to the reader in order for the rest of his story to work. Wilson himself explains that he was very suspicious of the institution from the beginning. His assistant Vincent Spaulding had to talk him into going down to apply for a position and then had to keep him from getting discouraged by the large number of other applicants and push him up the stairs and right into the office of his henchman, who also had red hair and called himself Duncan Ross. Even after Wilson had been hired, he still had misgivings.

Well, I thought over the matter all day, and by evening I was in low spirits again; for I had quite persuaded myself that the whole affair must be some great hoax or fraud, though what its object might be I could not imagine. It seemed altogether past belief that anyone could make such a will, or that they would pay such a sum for doing anything so simple as copying out the Encyclopaedia Britannica.

Doyle is assuaging his reader's skepticism. Once Wilson is assured and has started working, the reader is assured as well.
Sherlock Holmes was a gold mine for his creator. The great detective was featured in four novels and fifty-six stories. He was famous on both sides of the Atlantic. Doyle was one of the highest-paid writers of his time. But he was canny enough to realize that he needed variety. He couldn't just write stories about upper-class types who had some delicate problem and were willing to pay a lot of money for help. Doyle realized that by depicting his detective as a man who cared little about money and everything about exercising his mental powers, he could introduce a whole spectrum of characters and settings from top to bottom of English society.
Jabez Wilson is a good example of a client who has a problem but can't afford to pay the high fee a professional like Holmes would deserve. Wilson only netted about thirty pounds from his work at the League, and he doesn't want to part with any of it. He tells Holmes:

I did not wish to lose such a place without a struggle, so, as I had heard that you were good enough to give advice to poor folk who were in need of it, I came right away to you.

This is very important. It helps to establish that Holmes is good enough to give advice to poorer people. Many of the other Sherlock Holmes stories will get started by the arrival at Baker Street of a man or woman who needs help but can't afford to pay for it. A good example is the impetuous arrival of Helen Stoner in "The Adventure of the Speckled Band." Holmes often helps young women in distress. In another story, "The Adventure of the Copper Beeches," Holmes assists a young governess named Violet Hunter, and in "The Adventure of the Solitary Cyclist," Holmes saves the honor of an attractive young woman named Violet Smith. None of these clients can afford to pay Holmes for his services, and all of them have the common denominator of taking Holmes and his friend Watson to different picturesque places, sometimes clear out of the city. In "The Red-Headed League," the reader is given a detailed description of the neighborhood surrounding Wilson's pawnshop. The modern reader enjoys seeing the sights of England and meeting many of England's quaint Victorian characters.