why do some people go for tuition classes besides being weak in that particular subject?

Answer 1

If you're talking about Singaporean kids, it's most probably because their parents want them too. Many parents are afraid that their children will lose out to others and thus, prefer to send their children for tuition. Of course, not all parents are like that.
Also, sometimes people go for tuition classes hoping to get more notes/exam papers to do.

Answer 2

People take classes hoping to get better, to learn. If you only took classes based on what you are already good at, then you'd never excel, or break beyond what you are.