I do this a lot with my students, and there is a great reason for doing it. I have found with students that if I tell them before they go up that we're going to try a landing they have too much time to think about it. Then they start trying to remember everything, make sure everything is perfect, and get worked up for the landing. When it comes time to actually land they will usually be a total mess and nowhere near ready to land. On the other hand, I just let them fly approaches and I tell them to listen to what I am telling them on the approach. Then when they are lined up on a good approach I'll tell them to go ahead and let it set down and land. By suprising them with it they don't have any time to think about it and they land with no problems at all. After that it's usually a piece of cake, since they know that they can land the plane they don't get worked up over any more.

Ken