Hi -- Droopy makes a good point about getting to know your teachers. I love it when students drop by to chat. It's great when someone takes something from classs and applies it in their life, or notices something new, or remarks on some project.
I usually don't have time for long conversations -- unless I am helping the student with a project -- but these little check-ins are great.
As a matter of pure strategy, I find it very challenging to give a disappointing final grade to a student who has taken the time to make a personal impression.
I will never, ever "fix" a grade for a single student. That is unethical. But let's say I have three students who I know have been getting things out of the course but their grades don't seem to reflect what I know they've learned. I do have the authority to raise everyone's grades in the class by the same amount. (Students call this a "curve" but it isn't. This isn't the place for the technical explanation.)
Bottom line: Knowing students in my classes helps me to make better informed judgments about when my "evaluatory protocols" do not fairly and accurately measure the students' achievements. That is, my tests, assignments and grading criteria may not have been well enough explained or overly stiff.