I interpreted it as related to the Dunning-Kruger effect (http://en.wikipedia.org/wiki/Dunning%E2%80%93Kruger_effect http://en.wikipedia.org/wiki/Dunning%E2%80%

I interpreted it as related to the Dunning-Kruger effect (http://en.wikipedia.org/wiki/Dunning%E2%80%93Kruger_effect http://en.wikipedia.org/wiki/Dunning%E2%80%93Kruger_effect): students think that they understand things when reading the slides, but sometimes they don't realize it. Therefore, making it obvious that they cannot understand the slide can sometimes help. At least in some institutions, a teacher's duty also includes protecting the students from themselves and their own ignorance, by forcing them sometimes to go to classes. They want to minimize failure rates. Also, it should be noted that good students can often learn by themselves (using other materials), but it's the bad students which require more effort.