Leaving the jam to sit overnight before cooking it is a technique used to extract pectin from the fruit. Pectin is a natural thickener found in fruits and it helps the jam to set properly. Some fruits, like mixed berries, have a low pectin content and require additional pectin to achieve a thicker consistency. By letting the berries sit overnight with sugar, the sugar will draw out the pectin from the fruit making it easier to thicken the jam. This technique allows for a more efficient use of pectin and can result in a better texture and consistency of the jam.