Zakah: Great Wisdom and Many Reasons

Now a Muslim, why am I obliged to pay zakah? Why has it been prescribed at all? If it is about charity, shouldn’t I have the choice to give it or not?

Sadaqah: Its Virtues and Benefits in Qur’an and Sunnah

What is the difference between zakah and sadaqah? What are the benefits of sadaqah? How does it impact the giver and the whole society?