It is pretty widely known that Santa loves cookies. That jolly guy just can’t get enough of them. The origin of Santa’s love for cookies and why we leave them out for him is a little unclear. Here are the four best explanations for why we leave cookies for Santa.
1) As part of the Feast for Jolly Old St. Nick children would leave snacks out for him overnight. In the morning the snacks would be replaced by gifts.
2) Before Christmas trees many families had paradise trees. These trees were covered with fruit and baked goods. Santa liked to snack 
on these decorations as he traveled from house to house. As time progressed many families switched from edible decorations to decorations made of paper and glass. Without snacks to keep Santa energized he would become very tired so families started leaving him cookies made just for him.
3) In Norse Mythology Odin, ruler of the gods and Asgard, has an eight legged horse named Sleipnir who accompanies Odin on his 
Yuletide hunt. Children would leave snacks out to attract Sleipnir, hoping that they would stop by their house. Many Dutch families continued the tradition. This may also be the origin of spreading ‘Reindeer Dust’ in your front yard to attract Santa’s reindeer.
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4) During the Great Depression many families were left with next to 
nothing and barely had enough to survive never mind give out gifts. Many parents feared this would make their children greedy. In an effort to promote sharing parents urged their children to leave gifts for Santa.
The History Museum of Western Virginia 