Benefit of drawing 2 margins

Right side margin for rough work............if your rough work shows "I studant knows formula to solve the sum but he/she got wrong answer due to wronge calculation" then the examner can give you some marks on the basis of your rough work and examner having some words for explenation against given marks.

it is a matter of policy who follows...

let them follow the margin and get good margin of marks

All the best ..........