Why?
Look at it this way. The students are customers, buying their education. You are a purveyor of that education. Why should you not be evaluated by the customers, for any reason they deem relevant? If they feel that you are not giving them their money's worth, or feel that you gave exceptional value for their buck, does not evaluation literally mean assigning value?
They are simply stating their opinion about whether the product is worth the price.
“... students are customers ...”
IMO, this is the wrong analogy. Learning is a shared activity, and responsibility, in the classroom. Teachers are, at best, learning facilitators. As the old saying goes, you can take a horse to the water trough but you can’t make it drink.
Not really. Neither students, nor, actually, patients of a doctor know whether the knowledge/treatment being given or recommended to them is “worthy,” or really what it is worth. That is precisely why I have a Ph.D. and they don’t and why a doctor has an M.D. and I don’t. One of the problems with higher ed has actually been overkill of the “students as consumers,” which has driven from universities necessary and useful courses to be replaced with “feminist studies” and “wine tasting.”