Answer 1
To let Muslims practice their religion. And to pay a "non-muslim tax". Why ? Because Muslims pay a "muslim tax", the Zakat (money to the poor).
Answer 2
Answer 1 is a vast and inaccurate oversimplification of Dhimmi Law. A Dhimmi is a Non-Moslem who lives in Moslem-ruled state. The Dhimmi does have to the jizya, or tax for not believing in Islam. Dhimmi also had to pay additional taxes on land-holdings and these taxes where called kharaj. In addition, Dhimmi could not sell wine or pork in the public marketplace. They were not allowed to build new places of worship, restore old places of worship, or perform any act which could be viewed as proselytization. In many jurisdictions, a Dhimmi could not trust law enforcement to protect him, a judge to accept his testimony (especially against a Moslem), and in certain cases had their children stolen by Islamic rulers to be raised as Moslems (most commonly in the Ottoman Empire's devshirme system). The Dhimmi was a humiliated second-class citizen.
As for the Zakat, Moslems pay the Zakat as an act of faith and belief in the same way that Christians paid tithes to the Church in the Middle Ages. Paying the Zakat is a privilege and an honor and a pillar of faith. Paying the jizya and the kharaj are forms of repression. The money expended by each party is irrelevant, it is the purpose behind the tax that is important.