Allah made Zakah compulsory on the wealth of a believer as a test of their sincerity. The believer has full conviction that it is Fard and they know Zakah is the third pillar of Islam. It is a pillar of Islam- not a virtuous option without which a believers Deen will collapse.
Taken From the Book Minhaj al-Muslim Vol.2: From the wisdom of Zakat-ul-Fitr is that it purifies the fasting
soul from the effects of useless, non-beneficial speech (or acts) and obscene speech (or behavior). Likewise, it keeps the poor and needy people from having to beg on the day of Eid.
