If a market isn't nearby, citizens and workers will visit the closest stock pile to their home to collect the goods that they need.
Stock piles maximum storage is related to its size. You can place stock piles as large or small as you like. As a town grows, its storage requirements will also increase, requiring more stock piles.
The closer stock piles are to buildings that produce resources, the better. Workers generally have to transport goods from their creation location to stock piles. By kepping storage close, resource production rates can still be high when there are no general laborers to transport the goods.
Clicking on a stock pile will show the contents of the area.