One of the cardinal rules of landscape photography is to align the horizon with the frame. This is not so much a rule as an obvious choice. Why even consider doing anything else? Our eyes naturally perceive a horizontal horizon. But in the interest of best highlighting the colors, patterns, and objects in a scene, is this always the best choice?
Consider another cardinal dictum: the rule of thirds. Place the horizon at either the lower or upper third of the frame. Like the above guideline, this rule usually does yield the best perspective on a scene, and it's not hard to understand why. Usually there is a dominant element of the scene, such as an interesting foreground object or a dramatic sky, and it makes sense to give such elements a larger proportion of the frame. But if that's the case, then the rule itself is has no fundamental importance; it is merely a recipe for photos that look nice most of the time. The real rule is to pay attention to the relative placement of objects in the frame, and position them very deliberately to best convey the scene.
Given this, the horizon rule seems somewhat arbitrary too. The real goal is deliberation, not rule-adherence. I find that tilt can sometimes even provide a perspective more in line with what's observed than a horizontal one. In the above beach scene from Manzanita, Oregon, there are few vertical objects, so the eye has a harder time orienting itself. The bike tracks guide the eye from the corner into the distance. It was a very ethereal scene, and I think the tilt adds to that. It also highlights the otherwise sedate gradient in the sky by slanting it across the scene.
In contrast, the below image has many orthogonal features, but it too looks nice with some tilt. Here the railroad tracks guide the eye from the corner to the vanishing point. With the tilt, it is not simply an industrial part of MIT with the research fission reactor. The unusual perspective shakes us free of our preconceptions, letting us focus on the geometric shapes in the scene, the angles, the lines, and the parallelograms, and how they emerge from the vanishing point.