Layman.
The biggest reason why higher wages were paid by Ford wasn't so that the workers could afford to buy the cars. If this is the reason then why didn't all the companies do it? Not to mention that the American market had millions of people -- and Ford employed just thousands (~14 000 to be excact). Ford didn't employ enough people to make it matter whether the workers buy the car or not. It just doesn't make sense. This "higher wages to buy cars" thing is just a legend.
The real reason was that the job was incredibly monotonous and quite hard -- many workers quit after working there for a short amount of time. Teaching new people to do the work was expensive and took time.
Ford’s turnover rate was very high. In 1913, Ford hired more than 52,000 men to keep a workforce of only 14,000. New workers required a costly break-in period, making matters worse for the company. Also, some men simply walked away from the line to quit and look for a job elsewhere. Then the line stopped and production of cars halted. The increased cost and delayed production kept Ford from selling his cars at the low price he wanted. Drastic measures were necessary if he was to keep up this production.
Higher wages kept the people from quitting. In some cases raising wages can reduce the total labour costs.
Also:
The $5-a-day rate was about half pay and half bonus. The bonus came with character requirements and was enforced by the Socialization Organization. This was a committee that would visit the employees’ homes to ensure that they were doing things the “American way.” They were supposed to avoid social ills such as gambling and drinking. They were to learn English, and many (primarily the recent immigrants) had to attend classes to become “Americanized.” Women were not eligible for the bonus unless they were single and supporting the family. Also, men were not eligible if their wives worked outside the home.
I highly recommend reading Fordlandia: http://www.amazon.com/Fordlandia-Henry-Fords-Forgotten-Jungle/dp/0312429622/ref=sr_1_1?s=books&ie=UTF8&qid=1396471818&sr=1-1&keywords=fordlandia
Really good historical analysis that answers exactly the question you're asking. Effectively, Ford paid higher than average wages but also demanded incredible productivity and prescribed a particular lifestyle for his workers in exchange for the pay. There was also very little job security and his factories employed various people whose job it was to keep the employees in line - whatever that took. All of this was taken to the extreme in Fordlandia and it all sort of fell apart - the employees really didn't care what Ford thought they should spend their money on or how they should live their lives.