What is the purpose of having grades? What is the idea behind giving a student a grade? It might sound like a funny question but I'm serious. 

I ask because in my previous question about disputing a mark, it's hard for me to decide how many marks and how much effort I should put into having a mark changed. If the purpose of going to school is to learn (something presumably you don't already know) then how do marks fit into the equation?

Given some of the answers and comments on the question I'm curious, how did this mentality that marks are non-negotiable arise? There seems to be the belief the prof has a totalitarian rule over the students. This doesn't make sense, especially considering how commercialized some schools have become. In any other area of business if a client pays (e.g. a student pays tuition) and is dissatisfied or has a concern about a service, then the company would work with them and either explain or change some part of the contract. How is it with teachers they get away with simply saying "that's the right mark"? (I know that's rather a facetious example but hopefully the point is clear). I'm certainly not suggesting one can or should be able to buy grades: rather I meant in business there seems to be a certain level of diplomacy which doesn't exist between students and teachers. For example in the question linked to it was mentioned that even if the wording to a question is vague, if there had been examples in class of a similar question then it should be known what is being asked for. This wouldn't happen in business. If a client said they wanted x, they're not going to pay for y; conversely if they had asked for x and wanted y, they're not going to sue the company. 

It may be relevant to note that where I go to school, costs quite a bit of money (and it's a public university).

After reading this several months latter, I hope my tone didn't come across as too harsh.
A professor who I used to work with once gave me an explanation that I found quite useful for understanding the purpose and philosophy of grading.  Universities, he said, must always struggle with a tension between two educational goals, illumination and certification.


Illumination means the intellectual development of the student, bringing them a deeper understanding of a subject, its relation to the world, and the deeper issues it may touch on.  From the perspective of illumination, marks are intended to be feedback to students that helps them realize weaknesses in their understanding so that they can fix them.
Certification means evaluating a set of skills acquired by the students against an objective standard, to attempt to measure their fitness for certain tasks or professions.  From the perspective of certification, marks are intended to be objective judgement of the fitness of the student for carrying out tasks requiring the skills taught in a class.


These two are often in tension with one another because certification pushes teaching toward rote practice and standardized testing and grading, while illumination pushes toward more open-ended exploration and interactive formats which can deliver much more benefit for apt students but are often very subjective.  Most classes try to deliver both, to varying degrees of success, though some classes may almost entirely hew to a single side of the balance.

You need to decide what you're after from the classes that you take.  From your "pay for service" tone, it sounds like you want the career value that comes from certification.  But certification isn't valuable if the standard can be easily negotiated, and so professors have to set a standard and stick by it.  Sometimes they are even forced to by regulations.  From the perspective of certification, trying to negotiate for a better grade is trying to cheat the system and reduce the value of everybody's grades.  

Moreover, one of the "meta-skills" that is always being certified is the ability to figure out what somebody wants from you.  If you're out in industry and you deliver the wrong thing because you misinterpreted your client's needs and didn't ask for clarification, it will be difficult to argue that you should be given more partial credit.

If, on the other hand, you're after illumination, then grades are less important to begin with.  In that case, it's more important to understand why you got the grade that you got, so that you can improve your understanding of the material.  If you want illumination and you aren't getting it, you need to switch courses, majors, or institutions.