Writing science fiction and fantasy means world building. Maps, political systems, cultural details, biology, psychology, sociology, magic and technology are just a few examples of what the writer must manipulate. Certain aspects may be the same as our own world or differ in varying degrees. All the little details make a world rich and alluring, such as in Sir Terry Pratchett's Discworld series. It can also create a sense of alien 'otherness', such as in C.J. Cherryh's Foreigner series.
It's also fun to do. Removing restraint from the imagination is joyful and liberating. A child does not question the fact that her fantastical dragon is a different color every day and only comes out just after it rains. The dreamer does not question that someday, somehow, we'll live on other planets and encounter alien cultures.
The reader hungers for details, but beware the 'information dump'. Too much at once is boring and overwhelming, detracting from the story. A captivating narrative reveals its secrets along the way. But what if you need to reference a thing before explaining it? Steven Brust begins the Taltos series with such things, postponing their explanations, which adds to the pleasure of reading further. Tantalize the audience with the undiscovered (and take notes so you don't forget to make good on these promises ). However, don't leave your reader with too many unknowns; they'll forget half of them before you approach the clarifications.