The environment or landscape you set your story in has many uses. It can be an antagonist in ways, with difficult terrain for your characters to traverse. It will define what kinds of wild animals your characters might encounter. Deer or deer-like creatures would be found in a temperate forest, while deadly snakes would be more likely found in tropical forests or deserts.
Environment will also deem where people will be found. A desert environment will have people gathered around water sources. While a grassland environment might have people scattered throughout as well as centered around farms and cities.
It can also affect the way your characters travel. It might be easy enough to ride a mount or drive a cart across a forest path or over a flat plain, but what about in the mountains? If there isn't a pass or road through the mountains you can't ride, and even if there is your characters may still have to walk depending on the steepness and stability of the ground. If a mount can't keep it's footing it can be more dangerous than an enemy attack.
Even though it is only the setting of the story, the environment can play a huge role.