Adam Smith believes that the division of labor and specialization enables people to become adept at their job and therefore more productive. It is important to remember that Smith wrote before manufacturing was as automated as it is today. He writes:
The division of labour, however, so far as it can be introduced, occasions, in every art, a proportionable increase of the productive powers of labour.
When people are jacks of all trades and masters of none, they cannot produce as much as if they specialize. This is because in specializing, they learn how to do their specific part of the job quickly and well. Smith's reasoning calls to mind an assembly line pattern where each step in the process provides added value to the production of the final product.
Moreover, if the worker does not specialize, he or she does not learn how to use whatever machinery is required in the production process. With specialization, however, the worker becomes accustomed to the tools of the trade, so to speak, and with all the necessary machinery. Smith provides an example of the pin maker. He says,
the trade of a pin-maker: a workman not educated to this business (which the division of labour has rendered a distinct trade), nor acquainted with the use of the machinery employed in it ...
By becoming skilled and expert in the production of pins, the worker can out-produce another untrained worker who does not specialize. Smith says that without specialization, the worker:
could scarce, perhaps, with his utmost industry, make one pin in a day, and certainly could not make twenty.
With specialization, the work is broken down into specific tasks and the worker becomes adept at whatever small part of the overall manufacturing process he is responsible for. This accelerates the production of the end product. Smith notes that the work “is divided into a number of branches, of which the greater part are likewise peculiar trades." He describes the various tasks involved in the seemingly simple production of a pin. He notes that in some manufacturing plants, "all [work is] performed by distinct hands, though in others the same man will sometimes perform two or three of them.”
By this, Smith is also saying that in small factories that cannot afford to employ many people, workers can specialize in two to three tasks involved in the production process, but they must still specialize. The repetitive completion of two to three tasks will still enable them to become specialists.