Why do some instructors care so much about attendance?

It seems that in higher education, some instructors either do not care at all whether students attend, and others care minimally (often resulting in a 1-5% attendance grade or tie-breaker rule).

However there are also many instructors who seem to feel very, very strongly about attendance, and take it upon themselves to enforce this in various ways.

For instance, I was shocked by a recent question claiming that 15% of the grade would be lost for missing 2 lectures - according to the question author, the class meets so frequently that this would constitute missing less than 3% of the lectures! While this seems like an extreme case, it seems like it's not uncommon to find professors who may deduct 10% or so for missing a small fraction of lectures.

Why are these professors so preoccupied with making students attend? If attendance is so crucial to doing well in the class, wouldn't the students who don't attend do poorly in the exams anyway? Why additionally punish those students who did not attend, but did well regardless?

Answer

One theory is that it serves as additional motivation for students to attend class, which in turn helps increase their success in the course. It gives them a short-term incentive to do something which is hopefully also in their long-term best interest.

If attendance is so crucial to doing well in the class, wouldn't the students who don't attend do poorly in the exams anyway?

In many cases the instructor has found from experience that this is true. But the student (who has less experience) may not be as convinced.

Consider a student who wakes up in the morning and doesn't feel like going to class. In a class with no explicit attendance requirement, the student may rationalize: "I will just study harder tomorrow to learn the material that I missed, and I'll still be able to do well on the exam, so skipping class will have no consequences." But they overestimate their ability to do that, and end up not learning it as well. Or tomorrow they put off the studying until the next day, and so on, and fall behind. As an eventual result, they do not do well on the exam.

In a class with an attendance requirement, the student knows for sure that not attending class will have negative consequences. The biggest consequence (failing the exam) is very likely but not guaranteed, and the student may not be able to impartially evaluate just how likely it is. But loss of attendance points is guaranteed. So the student cannot pretend that skipping class is harmless. Thus the student is more likely to actually attend, which is in their long-term best interest anyway.

Hopefully, the ultimate result is that a higher percentage of students are able to meet the standards of the class. The flip side is that some students who have good attendance but poor performance otherwise may get better grades than they "deserve", but the instructor may feel that this tradeoff is justified.