What is the main function of Black Beauty in the story?

Black Beauty is the narrator of Anna Sewell's classic novel of the same name. He functions as narrator to show life from the horse's point of view during a time period in which animal welfare wasn't regarded very highly. By writing the novel from the horse's perspective, Sewell creates empathy for the plight of the horse at this time. She shines a light on their value to society, as well as their patient obedience in service to man.
The novel is set in 19th century England, well before the invention of the automobile. The steam engine locomotive was beginning to take the place of horses to some degree by this time, but horses were still a huge part of the everyday life of London. Some estimates say that there were as many as 300,000 horses in London in this era. Horses were used to pull carriages that served as taxis, hauled wagons with goods for merchants were the main mode of transportation and served in battle. Yet for all their usefulness, they were often not treated well or cared for properly. Anna Sewell highlights the plight of the horse in this era by telling the story through Black Beauty's eyes. She shows the kindness of some masters, the cruelties of others, and the ignorance of still others.
It is interesting to note that the first English law about the treatment of horses was the Treat of Horses Bill of 1821. This law sought to punish people who beat their horses. Although a step forward, the law didn't cover other forms of exploitation, such as in the cab scene in Sewell's novel where Beauty is used beyond his limits. The law was also difficult to enforce. To help with enforcement, a group called The Society for the Prevention of Cruelty to Animals was born. Below is a quote from Sewell's novel. It is the scene from chapter eleven where Black Beauty sees his old friend Ginger. When he last saw her, she was strong, healthy and beautiful. When he sees her in this chapter, she has been ill-used and badly neglected.

"One day while I waited for Jerry, a worn out cab came up beside me. The horse was a scrawny old chesnut. Her bones showed through her ill-kept coat. Her legs were shaky and her eyes dull. I was eating some hay when the wind blew a few straws her way, she stretched her thin neck and ate it. Then she looked around for more. I thought that I had seen this poor creature before. I tried to remember where. 'Black Beauty, my old friend,' she called. Is it really you? Do you not remember me?'"