To help isolate specific subjects from their backgrounds, one tends to use a wider aperture. This helps the subject be the main focus, instead of blending into the background. The aperture used would typically be f/1.8 or f/2, in comparison to f/13 or f/16 which would be a narrow aperture. Having a wider aperture leads to something known as “shallow depth of field.” This is more of a blurry background to help keep a main focus.
However, when photographing something as a landscape, a narrow aperture would be the better option. This is would help lead to a “good depth of field.” This would help keep everything in focus, rather than just one thing. With this, both the foreground and background would be properly focused in the frame. This is good when trying to keep all details showing.
