There is a huge difference between "knowing how to make a sailboat move" and "knowing how to sail". When you have new crew who can move a sailboat and will talk it up a whole bunch, you will want to try them out and see what they really know before putting them in charge of a watch cycle on their own.
Some of the issues that will come up during nighttime watches is the need to reef, and the ability to hold a course. Sure, when you are inland, you can sail all you want until the weather turns and just drop the sails at that point. In the ocean, it is not that simple and storms can produce much more powerful waves out at sea.
High winds and tall waves will make it harder to put in a reef, especially in the dark on a moonless night. To avoid this problem it is prudent to reef down at sunset so that if something comes up unexpectedly, you are already reefed and ready for it.
Reefing is great because it makes the mainsail smaller, but it still involves the boom. If you have inattentive crew at the helm, they might not notice that they have veered from course and about to jibe until the boom comes crashing over. Repeated powerful jibes can damage and destroy your traveler as well as damage the metal of your spars. It is best not to do this!
To avoid this problem, at night, we simply fly the trysail. It is small, our smallest sail and sail of choice for powerful storms; so we couldn't possibly reef down any further. It also negates the use of the boom. This means that if your new crew, or tired crew is not paying attention, a jibe is merely the flopping of a tiny sail with little load on it to the other side. No loud crashing or stress on your gear involved.
