What does "ha'Yom La'asosam" imply?
Rashi: It implies "ha'Yom La'asosam" 1 - 'le'Olam ha'Ba Lekabel Secharam'
Why is it important to know that Hashem rewards Tzadikim in the world to come?
Seforno: IN order not to lost faith if one sees that he is not being rearrded in this world for all his good deeds.
QUESTIONS ON RASHI
Rashi writes that Hashem pays the Rasha in his lifetime in order to make him lose the world to come. This encourages Resha'im - they will be serene in this world, and they are not concerned for what will be after!
So it says in Kidushin 39b - if one's sins outweigh his merits, Hashem will do good to him. He will be rewarded like (is proper for) one who fulfilled every letter of the Torah! 1
The Rasha is only concerned about this world, so Hashem pays him for the good that he does in the currency that he prefers. The Tzadik on the other hand, prefers to receive his reward in the world to come
Rashi writes that Hashem pays the Rasha in his lifetime in order to make him lose the world to come. This seems unfair (the world to come is worth far more than any reward in this world)!
Michtav me'Eliyahu (5, p.445): Mitzvos of a Rasha are merely a Kli for Ruchniyus, therefore their reward is in the framework of a Kli - in this world. Mitzvos of a Tzadik are a spiritual entity, therefore their reward is in the world to come.