Why should a Muslim man give dowry to a Muslim woman? What will happen if he doesn't want to give her a dowry?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

The husband is obliged to pay a dowry to his wife because Allaah ordered him to do so. Allaah Says (what means): {And give the women [upon marriage] their [bridal] gifts (obligatory bridal-money given by the husband to his wife at the time of marriage).}[Quran 4:4]. This dowry is exclusively the right of the woman. The majority of the scholars are of the view that the husband gives a dowry to his wife in return for making her private parts lawful to him. The evidence about this is the saying of the Prophet : "If the husband consummates the marriage with his wife, then he is obliged to give her a dowry for making her private parts lawful to him." [At-Tirmithi] Therefore, it is not permissible for the husband not to pay the dowry which is the right of the wife, unless he has a sound reason like being poor and unable to offer it in which case the woman may grant him a time until it becomes easy for him to pay it for her, like all other debts. However, the wife may renounce a part of it or all of it if she so wishes as it is exclusively her right, as we already mentioned.

Allaah Knows best.