This question requires an inference to determine the importance of the dinner as a prize. There are some possible inferences:
1. The group is going to make a pilgrimage of over 70 miles to the Thomas A Becket site and back. Food was an essential element in the early middle ages, and it was basically what people lived for because it was the source of energy they needed to carry on with their lives. Think about it: Compared to this century, food was not a pleasure, but a requirement as essential as water, air, and shelter.
This was also the feudal age in which "eating out" was certainly a luxury that many peasants (especially those in the party) could not afford. Even the rich at the time were experiencing a moment in history where the hierarchy was transitioning from the rich, pedigree-based families to families who had pedigree but no money. It would have been a treat to them as well.
2. The second inference is the fact that the group met at the Tabard Inn in London whose owner told them lots of happy and funny stories. Hence, he offered them the best he could give based on the above-mentioned circumstances: A free dinner.