Why is purifying the heart (Tazkiyah) so important?

Whether it is the eternal life or the transient life, it is evident that the way to success is through purification.

Purifying the self is one of the four tasks Allah gave the Prophet ﷺ, alongside reciting the Qur'an, teaching the Book, and teaching wisdom. The Qur'an gives the glad tidings of success to the one who purifies himself: "He who purifies (tazkiya) himself has certainly succeeded" [Qur'an, al-A'la 87:14], and the eternal gardens are named as "the reward of one who purifies (tazkiah) himself." [Qur'an, Ta-Ha 20:76]

Just as there are forbidden outward acts, there are forbidden inner states, jealousy, greed, pride, hypocrisy. The Prophet ﷺ said, "Allah does not look at the external outward appearance or the wealth, He looks at your heart and deeds." [Sahih Muslim 2564]

Some may think this inner work is merely optional, but Allah commands the believers to rid the inner self of all evils and purify it: "And whoever purifies himself does so for the benefit of his soul; and the destination (of all) is to Allah." [Qur'an, Fatir 35:18]