When work is marked, it usually only includes a single grade with a sentence of constructive criticism. However, the benefits of using categorised marking ensures that the student is totally clear on where his/her strengths are, where they need to improve, where they lack understanding and any parts of the exam criteria that they have missed.
Ah, the dreaded red pen. Students, parents and SLT often cringe at the sight of the red ink used by a teacher. Maybe because it's a symbol for blood?
I think it's more likely to be linked to the idea that teachers from a previous age would use just a red pen to mark work and would be likely to mark negatively; pointing out mistakes or circling poor work.
However, when a teacher marks their books now, they are usually (or hopefully) better trained into what marking works and what will benefit their students' learning.