Answer

A Kudoku or benefit is the effect of a meritorious cause.

The third chapter of the Sutra of the Infinite Senses, entitled the Ten merits, explains in detail the benefits inherent in listening to the Lotus Sutra even once.

I'm only going to mention the first of the ten benefits here.

"The Buddha taught the following.

O good men! First of all, this sutra gives rise to such a mind in those who have studied the Mahayana teachings but have not yet given rise to the mind of acquiring the Buddha's wisdom.

Similarly, for those who have no desire to make others happy, such a spirit of compassion will appear.

On the other hand, those who enjoy causing suffering and killing living beings will have a heart that wants to save them from their suffering.

Those who harbor envy and hatred towards those who are better or happier than they are will receive a rich heart that can rejoice with them in their happiness.

Those who are too attached to their possessions will develop a detached heart that will enable them to get rid of these possessions whenever they need to.

Likewise, for those who are too greedy and covet the goods of others, the spirit of charity will appear.

Those who are vain and proud of themselves will realize their inadequacies and errors, and will have the spirit to practice and firmly observe the precepts taught by the Buddha.

Those who are accustomed to selfish anger will also develop the spirit of patience in all things.

Those who neglect their mission, neglect or stray from the path will have the spirit to walk in a single stride along the path they should be following.

Similarly, for those whose minds are disturbed by the changes around them, a calm and stable mind will always appear, because they can see the universal truth behind the changes.

In the same way, for those who are stupid and can only see what is in front of them and cannot discern what is behind, the wisdom to see through it all will appear.

Those who have never had the spirit to save others will have the spirit to save others.

Those who do ten kinds of evil deeds will have the spirit not to do those evil deeds.

For those who seek only the happiness of visible phenomena, the spirit of seeking the true happiness that lies beyond phenomena will appear.

For those whose faith is waning, an unshakeable faith emerges that does not retreat a single step.

For those who do things according to their evil passions, the spirit that does things away from evil passions appears.

For those who have a lot of bad passions, the spirit that gets rid of bad passions will appear.

Good men. This is the primary benefit of this teaching, its inconceivable power".

As far as Nichiren Daishōnin is concerned, this is the definition he gives of kudoku. In the Oral Transmission of the Doctrine, he states:

"Kudoku) are the attainment of Buddhahood from this body, it is the purification of the six roots. Know that practicing in accordance with the words of the Lotus Sutra is the purification of the six roots".