Normally bees build up sufficent food storage during summer and would not run out of food.
During the warmer time of the year bees collect nectar and pollen when available, and what is not consumed during this time gets stored in their nest in honeycomb until all their storage space is filled up.
Bees use this surplus during times when there is not enough nectar and pollen to collect, i.e. during winter, on cold or wet days or during a drought and even during summer with normal weather conditions when there is insufficient nectar and pollen to collect.
However, when beekeepers rob bees of their "surplus" honey before winter, the remaining storage might not last them through the winter. Likewise, bee colonies that could not collect sufficient stores during the season might be short of food stores when the season finishes and need the beekeeper to feed them.
Feral bee colonies with insufficient food stores at the end of season will not survive the winter and disappear quietly and unnoticed - nature is not always kind.
As a beekeeper, whenever your bees are short of food, at any time, not only before winter, you need to feed your bees to prevent them from starving!