“How important for the narrative are the descriptions of the storms?”
In Robinson Crusoe, the storm is much more important than it seems. It gives the reader a sense of foreboding and disaster and of fighting against the inevitable. In Cast Away, a modern interpretation of the book, the airplane flies into a storm. The watcher (it is a movie instead of a book) has the slightest spark of hope that they can avoid the storm and make it out because we know how deadly the storm is and if they cannot make it out, there is no hope. Unfortunately, they crash into the ocean nonetheless. The storm also has a practical aspect to the story. In Cast Away (and in Crusoe), because of the storm, the plane is taken far off course and no one knows where the wreckage is or even if anyone survived the crash. So we can see that the storm is an extremely important part of Robinson Crusoe and without it, the story would not have the same influence over readers that it does.