It helps one to surrender any selfish motive and to dedicate actions and thoughts to the absolute.
Karma Yoga is a path of devotion to the work. One looses his identity while working, only selfless work remains. This state is very difficult to achieve. Generally some rewards or incentives or outcome follows the work and one is attached to this reward or incentive. This is not the Karma Yoga. Non-attachment with the work and becoming the perfect instrument of the super consciousness in this manifested universe is the ultimate aim of Karma Yoga.
In the initial stages of Karma Yoga, individual possesses strong sense of ego and consciously or unconsciously he is attached to the fruits of his efforts or at least praise or recognition but by continuous involvement in the work and change in mental attitude, one can surely disassociate himself from the ego and his own personality. In this state the work becomes worship to the God, it becomes spiritual, also the individual becomes expert, skilled and Yogi. He achieves stability of mind in all conditions, he is not disturbed or excited or happy in any of the situations. He becomes divine & his actions represent God's will.
The essence of Karma Yoga as extracted from 'Bhagvad Gita' says: The world is confined in its own activity except when actions are performed as worship of God. Therefore one must perform every action with detachment. Buddha says: there are ONLY two reasons behind for all kind of sadness, pain and stress, which are desire and expectation.