The measure of all perfection is Hashem. The more like Hashem a man is, the more perfect he is, and the closer to Hashem he is. The Torah is the system of spiritual perfection. Its statutes bring a person nearer to Hashem by making him more like Hashem.
Hashem needs nothing and does nothing for Himself. This entire Creation and everything in it was fashioned to benefit others. Hashem is the Benefactor. And so, the more of a giver a person is, the more like Hashem he is, and thereby the more perfect.
There is, however, one critical observation. Hashem is very good at doing that which Hashem does. Hashem doesn’t need help in running the world. If Hashem so wished it to be, there would be no poor people, and the very concepts of poverty and wealth wouldn’t exist. Hashem created a world with different sorts of people – some strong and some weak, some brilliant and some not, some wealthy and some poor. And it wasn’t by accident. Hashem created the world this way to allow people the opportunity to use their strengths and talents, their resources and situations. If man does that not only does he grow, he is credited with the accomplishment.
If a wealthy person sustains a poor man it is considered as if he gave life to the person. Even though its Hashem’s world, and even though Hashem gave the wealth to the rich man, if the wealthy man gives to the poor man he is credited with saving him.
This seems to be the answer to the Rashi.
Granted, the kohanim bringing the avodah is a spiritual act of extraordinary impact. But the source and the measure of all holiness is Hashem. The way Hashem shows Himself in this world is through giving. When man gives to others he is acting as Hashem, and so there can be nothing holier.
The Torah is teaching us that when you leave over your leket and shichkacha you are acting like Hashem as much as you can. When I take care of another’s needs, that is the highest level of spirituality. The more we are focused on the needs of others, the more like Hashem we become.