Amina Inloes, Amina Inloes is originally from the US and has a PhD in Islamic Studies from the University of Exeter on Shi'a hadith. She is the program leader for the MA Islamic Studies program at the... Answered 2 years ago

The short answer is, it is prescribed by Quran and hadith.

A longer answer: In my view, part of the wisdom is that... sadly, the human being values what they pay for. Most people who pay $100 for a book will read it. Whereas most people who download books for free will never read them at all. A person who pays $1000 for a book will memorize every word in it!

While I do not want to suggest that a female is like a product that a man buys, it seems that, for some men, making a financial commitment towards a marriage is a way of demonstrating their overall commitment to and investment in it, and commitment is one of the necessary ingredients for a lasting marriage. (Whether that financial commitment be made in the way of mahr, gifts, money towards a home, paying towards the wedding, etc)

I am not saying all men are like this, it just seems to be part of how some people's minds operate. Of course two people can be genuinely committed to a marraige even if neither of them has anything financial to contribute. 

Of course mahrs should be moderate and not extravagant and should be according to one's capability.

Also in reality although there are theoretically equal opportunities for men and women, women are still more disadvantaged and paid less in the workplace, and are in more vulnerable situations when they have children, especially if they end up as single mothers. So anything that can help add some "insurance" to the woman's situation is not a bad thing.