Why that time period? Why that race? Why that land? Why that day, that moment, that person, that plot, why why why why?
"Does it really matter that much?"
Not every single time, yet even then, you still have to answer "why not?" Why doesn't it matter?
Take my own novel, Luna the Lone Wolf. Here are some questions I've had to answer over it's development.
It fits perfectly with the theme of being a "lone wolf". Wolves are by nature family oriented. Unlike us humans, they're simple at their core, but expand greatly once dug into. Really the fact is humans couldn't tell the story well enough. Not without adding too much into it. Humans have politics, money, technology, and so many other things we can't get away from. Wolves have all the emotional traits I need without all that, allowing the emotion of the story to be told totally bare. Plus being wolves, an animal I was passionate about anyway, it was easy to get excited about the research aspect of things.
Why a forest?
Didn't like the idea of wide open plains. The ecology of a forest gave me a few more options than other lands would have done. In this case, it just felt right.
Why a mocking bird?
I needed a bird that could believably make sounds wolves could understand. Being a breed known for making a vast range of vocalizations, as well being found pretty much anywhere, he fit the bill perfectly. And by the way, parrots were rejected immediately. I did not want Iago (from Aladin) in my story, thank you very much.
Why the time period?
Well the time period really didn't matter since this is all about the wolves. However, since humans were going to be there, might as well keep it current to help the human readers connect with things. After all, modern humans understand the idea of talking to a "rock". If the humans were just standing there, talking to no one (I.E. internal implant), it might be hard to have the wolves see it and still tell the human readers what's going on. I mean, try describing the Millennium Falcon entirely from a wild wolf's perspective and still have the reader know what it is he's looking at. The moment you try to add some kind of cute description using the thing's name, you've destroyed your perspective.
So on, and so forth. Every detail has a reason, even if the writer himself is not aware of it. Luna lives near a river. Why? It gave me a way to do things the story requires much easier. Why write only from the wolves' perspective? I tried including the humans, but they damaged the story. There are events that happen to him that also have their reasons, but I can't even post the questions without major spoilers.
The fact is, the writer should always ask each detail what their purpose is. When I get stuck in a section, I always go back to "why is this here? Why does this need to happen?" Often the answer leads to the fix. The why is the whole reason the story was born. It drives all that goes before and is left behind. I wanted to tell a tale about a loner facing his life as one, and the myriad emotions that brings with it. I wanted to share his pain, and his joy, with all of you.
You see, from that core reason is where I build everything else. From Luna being a wolf, to the bird, to the forest, to everything else that builds his world and his life as it must be told.
Ask yourself, why was Star Wars done in "a galaxy far far away?" I can think of a few, but I'd like to hear yours. Think like a writer. Might be fun. Why set this intricate, and complex universe so far outside of our own? Why not just send it a few hundred or thousand years in the future like Star Trek? Why?