Honey bees may need feeding for a number of reasons. The good beekeeper will make sure their colonies have adequate stores at all times. This should be assessed at every inspection and on a regular basis when not in the active season. Poor nutrition is damaging to a colony, the effects can last for some time.
There are a number of reasons why a colony needs feeding, including to supplement their stores for winter, to avoid starvation, to build up a small colony or to stimulate it in the early spring to take advantage of an early crop.
In the UK oilseed rape (canola) falls into this category. This is a delicately balanced problem, as it can often happen that the bees are built up into large colonies, then the weather turns cool and the bees are unable to visit the crop. Under these conditions the crop itself does not yield much nectar.
This results in the beekeeper using feed to build them up, then having to continue feeding, just to keep the artificially enlarged colony alive.
In some areas of the UK it is common for there to be a 'June gap' to occur. This is in areas where the early nectar flows have ceased and the later summer ones have yet to arrive. Some large colonies will need heavy feeding in order to survive.