The Custom Gradebook Columns API gives teachers/admins a way to add additional "notes" columns to the gradebook. It'd be amazing if there was an extra option when creating a column that made it visible to students and not just teachers. In large classes, having a generally visible text-entry column is invaluable for communicating certain kinds of information, like letting students know who their TA is.
This is similar to the long-standing Import Grade Comments. However, comments aren't immediately visible to teachers or students in the Grades area. You have to click to expand the comments before viewing. Whereas the notes column has information visible at-a-glance to teachers (and should for students, as well!).
It's all about making it easier to communicate with students. Custom columns already exist, so maybe it's not too hard to just add an API option to display them to students.