My simple understanding is as follows:
 MP is the representative of his constituencies. His vote speaks for the people who vote for him. We can see in US or UK that senators or MP would not vote simply it is party line. They vote in accordance to their constituency. For example, I could be a right-winged conservative, but my constituency is gay by large majority. When I am called to vote, I have to respect my constituencies’ wishes. If I don’t subscribe to this “democratic process” then I have to quit as MP or go elsewhere and win others vote to be an MP.
 The vote count is a statistical and historical record of the decision making process. The ratio of the majority will be a precious guide to how policy is implemented and how future decision making would to be made. For example, if a bill is passed with large majority then we know it is with the people, and it can be implemented with higher conviction and pace. However, if a bill is passed with slim majority, it would have to be executed with great compassion, care, circumspect, gentleness etc, because there is still a large minority who think otherwise.
 I think it is inconsistent when a person debate against a bill, and voted for it eventually. I find it hard to understand and reconcile. For example, if a MP were to speak against building of Casino, he has to vote against, when the vote is being called.
Can you write something to educate me, because I find that Singaporeans need to know more about how it works and the principal behind this policy.
You are right about the practice in Singapore and in the USA. In Singapoer, the MP has to vote according to the party line (unless the whip is lifted - which is seldom allowed). In the US, the Congressman votes according to his personal conscience (usually guided by the views of his constituents) and is not bound to vote according to the Party line.