Banting consulted William Harvey (an ear surgeon) due to deafness, and Harvey diagnosed that Banting's deafness was due to fat deposits in the eustachian tubes. He prescribed a weight-loss diet based upon Dr Claude Bernard's Diabetes diet, which cut out bread, butter, milk, sugar, beer and potatoes.
Harvey believed the diet would reduce corpulence as well diabetes. And to some degree it worked for Banting, who reportedly who lost 46lb in under a year and regained the ability to descend stairs normally. He also gave up wearing his truss and his sight and hearing improved. How factual that is we'll never know, what we also won't find out is the long term issues he suffered due to the diet, or how long he maintained his loss for prior to his death in 1878.
Banting was eager to share his success with others, and did so by publishing the short pamphlet, 'A Letter On Corpulence'  in 1863. Banting described obesity in much the same way that is viewed socially to this very day.
"Of all the parasites that affect humanity…I do not know of, nor can I imagine, any more distressing than that of obesity."
In the pamphlet he describes the distress he suffered due to the,
"...sneers and taunts that obese people suffer in public spaces."
So it seems that like modern DIEt's, Banting's was simply attempting to escape the social stigma attached to obesity and conform to social expectations as well as regain his health.
As I've discussed previously that diabetes is more to do with the inhibition of glucose via the randle cycle, due to excess of fats, particularly polyunsaturated fatty acids.
"More important than the physiological vacuity of a simple glycemic measurement was the ideology within which the whole issue developed, namely, the idea that diabetes (conceived as chronic hyperglycemia) is caused by eating too much sugar, i.e., chronic hyperglycemia the illness is caused by the recurrent hyperglycemia of sugar gluttony. The experiments of Bernardo Houssay (1947 Nobel laureate) in the 1940s, in which sugar and coconut oil protected against diabetes, followed by Randle's demonstration of the antagonism between fats and glucose assimilation, and the growing recognition that polyunsaturated fatty acids cause insulin resistance and damage the pancreas, have made it clear that the dietetic obsession with sugar in relation to diabetes has been a dangerous diversion that has retarded the understanding of degenerative metabolic diseases." 
Breads, other grain products tend to be more problematic than actual sugars (fruits, simple sugars etc yet despite this they continue to be promoted over sugars. It is no wonder Banting was successful, (depending on how you measure success) I have seen many people who have had great 'results' initially from cutting out potato, sugar, fruits etc (as I did myself in the paleo years). Only to suffer serious side effects as time passes and health declines. They also tend to realise that the weight loss was water and muscle, and the disasterous effect it has upon thyroid health, blood sugar regulation and immune function, to name but a few, just wasn't worth the quick 'fix' on the scales. We can learn a lot from these early pioneers, and I think Banting is a good example. He was passionate about his belief, wanted to help society and spread the word. Unluckily, he had no background in science so was not to know the damage his work would cause in the future. The money the pamphlet raised was donated to charity, which isn't the case for those that continue to profiteer of his work.
1. A letter on corpulence
2. Glycemia, Sugar and Starch in Context. http://raypeat.com/articles/articles/glycemia.shtml