As a rule, a large Focal Length is used to visually draw distant objects closer. Since these objects actually lie far away from the viewer and only appear closer the perspective effect on the object is reduced. Hence, extremely long focal length settings can cause an object to appear isometric.
Short focal widths have a different effect. The field of view is expanded and a larger area of the scene comes into view. This can, for example, be useful when creating interior shots of a room in which the distance between the camera and the objects in the room are very small. An extreme setting would end up looking like a fish-eye view. Be careful not to cause the view to be distorted, as demonstrated in the image below.
At the top of the image above a simple grid comprised of narrow cubes is shown with a Focal Width value of 11. At the bottom is the same scene, distorted as it can happen with a real-world lens. This type of curvature cannot be simulated precisely, in spite of a small Focal Width value, and would have to be added in the post-production phase.