System saves user preference
The user, who is an instructor, has the Gradebook page open and is copying and pasting students' mid-term exam grades from her spreadsheet. 
The user would like to see all 43 students in one page, so she clicks on the dropdown "Show x per page".
She figures 50 would be a good size to display all of her students in one page. She selects 50, and the page reloads. 