Why do beekeepers have to feed bees?

Why do beekeepers have to feed bees? 

Two answers to that.
1) Wild colonies have been decimated by disease, monoculture farming practices and massive use of Pesticides, such as the Neo Cotinoids. So evidence from research points to an alarming decline in wild colonies many of which are carrying American Foul brood.
Honey bees are endangered by the actions of man and this is not an exaggeration.
See Donald Simms, 60 years with the Bees.
A World without Bees - Allison Benjamin

The Beekeepers year
Answer 2

Five to ten years ago you would have been almost correct wondring why we feed bees. 

Removing the 🍯 honey crop
The Autumn Management Regime

After the honey crop 🌾 is removed a series of management actions are carried out by the Beekeeper with the welfare of the Bees in mind.

Although, to qualify the comment about five years back, it was even then normal to feed four successive gallons of 1:1 sugar syrup, after the removal of the Verroacide Vetinary Medicine of course and to get that done from about 21st August and completing the rounds 4 times before Autumn temperatures dropped at the end of September.

Liquid freezes so you cannot feed liquid feeds in freezing weather.
This is a lot of work.
So off comes the crop. Now there is no honey 🍯 on the hive for human consumption.
There is about 50kg of honey in the brood chamber for the bees winter stores and that is left on.
By Spring they will have burned up that honey and starvation becomes a real risk - even if the population drops from 50,000 down to 8,000.

Verroa medicine

Verroa destructor is a mite thst sits on the honey bees back. It has a spike which it sticks into the honey bees kneck and sucks out her blood. In so doing the mite passes numerous Viruses which soread through the colony and can kill the whole colony. 

On goes the verroa medicine for 14 days.
Off comes the verroa medicine.

Sugar syrup feeds.
Now 4 separate visits, once a week, are required by the Beekeeper to put on a gallon of sugar syrup a week and that takes 4 weeks up to the target 🎯 end of September.

That used to do the trick and you would go back around your beat to remove all of the Autumn feeding regime equipment.
You only had to go back in November to give 2kg of Fondant. Then on  Boxing day 2kg Fondant.... Leave until February was the  received wisdom and feed last block of 2kg fondant and then wait until after March 10th to feed 4 separate gallons of sugar syrup as you did in the Autumn.

Spring Management Regime
As the Spring weather improves the Beekeeper watches as Katkins ( Salvia), snowdrops, Spring blooming 💐 flowers and the beginning of the sequences of 🎄 Tree Pollens began to flow. You knew you bees were safe when the Dandelions bloomed ( massive sigh of relief)

Climate change has changed everything.
Beekeepers leave still leave substantial amounts of honey, up to 50kg, in the brood chamber for the bees.  But.........

Climate change
The planet is almost 2°c warmer than it was.
In the past harsher winters kept bees largely in the hives and only coming out for defecation flights and water collection on nice sunny November mornings perhaps when the temperatures allowed, briefly above 8°c.
When bees fly they use up energy. That energy comes from the honey 🍯 left in the Brood chamber by the Beekeeper.
Personally, I noticed a more marked change from Autumn 2015.
In 2016 Spring was damp and rainy and temperatures did not rise above 14°c until late May.

Spring Dwindle caused by starvation

These longer wet Spring seasons have now become the norm and the bees cannot get out to collect pollen and nectar and so again they burn up even more stores.
They burned up nearly everything in the Winter and there's not a lot left but the Queen is now laying again, the population is rising by up to 4,000 every 21 days and there is a shortage if stores in the hive.

Whereas in the past they were mainly in cluster in their hive and CONSISTENT and longer cold spells kept them confined to the hive we now see much milder inconsistent weather which permits the bees to fly, using up their food stores whilst they fly looking for non existent pollen sources in Winter time.

Continuation of laying by the Queen.

On top of this extra burning up of stores, the milder weather sees the Queen continuing to lay eggs and increasing the size of the colony instead if reducing it in Winter time
More larvae = more bees = more stores consumed.
So the colony is squeezed for stores in the Autumn and again in the Spring.

I have laid out some small part the work in a  Beekeepers year. I have also explained why we are forced now to feed our bees or see them die.
Does this answer your questions?

If you are wishing to learn just how close to a world wide Calamity in food production we are please read
A World without Bees
Allison Benjamin