Khums is the right of the Imām (as) and of the Sadaat. Therefore, it is paid in accordance with the following ayat of Qurān:
Know that whatever benefit you receive, one-fifth is for Allah and the Messenger, his close relatives, orphans, the poor, and the travellers, if you believe in Allah and what We revealed to Our servant on that decisive day when the two armies met. And Allah is Most Capable of everything.
(Surah Anfal, verse 41)
As such, khums is an act of worship and must be given with the right intention, i.e. for attaining proximity to Allah (qurbatan ilallah).
If khums is paid correctly, then the earnings of the person are purified and lawful (halāl). On the other hand, if khums is neglected, the earnings of the person will be regarded as ‘spiritually impure’ and can invite divine retribution in this world and the next.
Without khums, many important projects and many poor Muslims around the world would not receive the help they critically need.