In most cases that I have limited fps, it was to keep things from moving too fast.  There are of course other ways to do that without limiting the fps, but this is a much simpler way to do it.

The other thing to consider, is that there is an actual maximum fps that the human eye can detect (it is somewhat higher than the 25 fps previously believed, but it is still a proven phenominum.  I kind of remember reading somewhere that the human eye can detect subtle differences up to 70 fps or so).  So as long as you are limiting the fps to a value higher than that, there should not be a noticable impact on smoothness; only on speed (which, as I mentioned, is generally the factor you are trying to limit).

One more reason is to release CPU time for other apps...

      //tool.checkForCollisionSpherical(newSimpleVector(0,(tool.getTransformedCenter().y*-1)+30,0),100);
int obh = tool.checkForCollision(new SimpleVector(0,2,0),(tool.getTransformedCenter().y*-1));

import com.threed.jpct.*;

class AddTriTest {

   public static void main(String args[])throws Exception{

   Object3D ob = new Object3D(30);
   ob.addTriangle(new SimpleVector(0,0,0),new SimpleVector(0,100,0),new SimpleVector(100,0,0));
   ob.build();
   
   World world = new World();
   world.setAmbientLight(200,200,200);

   world.addObject(ob);
   world.newCamera();
   
   Camera cam = world.getCamera();
   cam.moveCamera(Camera.CAMERA_MOVEOUT,200);
   
   FrameBuffer fb = new FrameBuffer(800,600,FrameBuffer.SAMPLINGMODE_NORMAL );
   fb.enableRenderer(IRenderer.RENDERER_OPENGL);
   fb.disableRenderer(IRenderer.RENDERER_SOFTWARE);

   world.removeObject(ob);
   ob.addTriangle(new SimpleVector(0,-100,0),new SimpleVector(0,0,0),new SimpleVector(100,-100,0));
   ob.build();
   world.addObject(ob);
         while (!org.lwjgl.opengl.Display.isCloseRequested()) {
         fb.clear();
         world.renderScene(fb);
         world.draw(fb);
         
         fb.update();
         fb.displayGLOnly();
         Thread.sleep(10);
      }
      fb.disableRenderer(IRenderer.RENDERER_OPENGL);
      fb.dispose();
      System.exit(0);
   }
}