During his first voyage to America, Christopher Columbus kept a daily journal in which he recorded what happened in order to present the completed book and other writings to his investors in Spain. He makes entries every day describing anything noteworthy which happened aboard his ship. These events include astronomical observations, distance traveled, the state of the crew, and any sightings of animals or debris in the water. Over the course of the voyage, he reveals how the captain lies to the crew, overestimating the distance traveled each day. Eventually they start complaining about the length of the trip, until the discover the deception and threaten a mutiny. In just the right moment, they hail land and start preparations for disembarking. They encounter some Native Americans and a great deal of potential natural resources in America, all the while thinking they have found India.
Since he is writing for monarchs, Columbus takes great pains to ensure the journal is as worthwhile of their time as possible. More importantly he is looking after his funding by keeping this journal as a sort of evidence of the value of the trip. As a businessman, Columbus is looking forward to more voyages to the New World in which he can potentially make more money. The journal is the best method with which to entice the King and Queen of Spain to fund more expeditions, by describing all the resources which may be gained from the new land. Basically it's a marketing tool.
For his part, Columbus is interested in fame and fortune. He definitely desires wealth from exploiting the New World, but he also wants to be recognized as a famous explorer. Most likely this is the explanation for a lot of the early logs in which he exaggerates the dangers and trials of the voyage in order to make his accomplishments seem greater to the reader. This is not to say that the trip was easy, but Columbus certainly dwells upon his suffering more than is strictly necessary. In any case, his work was a success, and he did become a celebrity back in Europe.
