In my opinion the change in perspective makes it more real and more visually familiar. Most of my encounters with trains took place with the locomotive looming over me, rather than spotting from a bridge or other overhead location. So it's a more familiar angle to see it from, which adds to the realism and immerses you more into the layout.
When I was running just a simple loop of track on the floor (hoovering up who knows how much dog hair and carpet fluff into that old engine's poor motor) I would lay down flat on the floor, one ear to the ground, and watch the train roll past. It was a magic moment that just isn't the same when you're watching from a chair or standing.
