I used to teach at least one of the first semester courses each semester, so I always knew where the students' knowledge and skill levels were. Lately, I've been teaching upper level courses exclusively, so I have to use a Pretest to see what was learned in first semester. Sometimes, I'm really surprised at either 1) what the students didn't learn or retain, 2) what may have not even been taught to them, or, worst case scenario 3) what may have been taught to them INCORRECTLY. Having a solid Pretest helps me and the students address so many issues, challenges, and potential problems before these things even arise. That way, class can be smooth and productive without anyone getting too frustrated.