The more in-depth and details the author gets the more of a new world they have created in your mind. They take you away into a whole other universe. Yet a great author won't stray to far that you get lost and don't know how or where you got there. 

Yes, most science fiction stories are things that are highly unlikely to happen anytime in our near future. But there is a fine line between being completely unrealistic and somewhat believable. Most science fiction books do this by incorporating daily live things and then adding in their own twist or unrealistic type flair. That is what makes each book unique and original. 

Having a purpose is really key in every book genre. But in a science fiction it is a must. An author doesn't want their readers to finish the book and wonder to themselves, "What was the point of that?" They want you to feel a completeness and the satisfaction that there was an issue and that issue was solved by the end. Also they want you to come back and tell others about their book. How are you suppose to tell someone about a story if there was no real purpose to it? 

No one wants to open up a so called 'Science Fiction' book and start reading it and it turns out to be a book about poetry. That just doesn't work. So to make any science fiction into a great science fiction it really needs to be one. This may seem like a given but you would be surprised at how many so called 'fill in genre name' books are out there and they end up being about something totally different.