Again – novelty. We like a predictable routine but we also like variety. Variety is the spice of education. Anything you do the same way every single day has no novelty and therefore little student excitement.
Intermittent Reinforcement. It locks in the behaviour or the knowledge.
Apparently rats will continue to press a button if it only rewards them a few times, in the “hope” that they will strike it rich again. This works for gamblers as well. Getting the reward not every single time is a huge reinforcer of “trying again.”
You still get a lot of extra writing. If you do it for 2 weeks once a month, for 8 months, for 20 minutes a day, from grades 4-8 students will have done 10,000 minutes of extra writing, in addition to their all their regular writing lessons.