A reply to /u/Rotundus_Maximus

Navigation charts, no matter how crude (and many of them were plain incorrect) serve many roles. Before the journey even begins, they allow you to do some level of planning, by estimation of distances and thus time required between stopping points where you can replenish supplies. They give an idea of primary and secondary objectives for exploration. They often inform of hazards such as stormy locations, strong currents, contrary winds, etc.

Finally, they are a useful tool for convincing sponsors to take the risk. Columbus famously insisted on an incorrect map and globe to convince himself and some of his supporters that the earth was smaller than it is, and that China/India were farther east than they are. Ergo, both are reachable by sailing west directly.

In execution, charts show landmarks that can be used for navigation and calibration. In that era there was no reliable way to estimate longitude, so this is key.

Of course, they are useful for revision, such as when de Balboa encountered a great ocean west of the Central American isthmus. Surely then, Cathay isn't as near as Columbus had insisted.

I hope this isn't a homework question because it sure reads like one.