As discussed above, using splines to create a canyon or river valley is a useful technique. But there are other uses as well. If you have a slope in a urban area that needs to be graded to a even slope, lay a few road or rail splines from the top to the bottom and use smooth spline, and you can get a smooth slope without having to use any of the terrain tools.
Also, I built a ski resort on a route and to get a somewhat realistic consistent gradient for each ski run, I used rail splines (instead of road splines) because rail splines allow you to set the gradient. Smooth the splines, move the spline points slightly, then smooth again. The result is a realistically wide ski run with a slope that can be beginner, intermediate, or advanced. In that case, I used invisible track which I left in place so I could see in surveyor where my ski trails were.