Extremely high scoring: If you get your maths optional right then you will make it to the final list.
Certainty: If you have attempted your paper well then you are sure that you will get good marks. For example this year just by attempting 400 marks paper you could get a decent 260+ marks. Even if you don't get good marks in first attempt you can be sure that you will increase your marks in subsequent attempt.
Fun: Mathematics is a delightful subject and therefore doing maths takes you away from somewhat boring humanities.
Good Impression: The fact that you have taken Maths makes a good impression on interview board members. They are very pleased to see that you have opted for a tough optional.
Easy paper: The difficulty level of paper is quite moderate and almost all questions are directly picked from the standard textbooks.