“With one action, a person changes only a small part of reality.
When he waves the lulav and etrog, he cannot transform all of the etrogim in the world into etrogim used for a mitzvah.
Nor, when he writes a Torah scroll, can he transform all of the animal hides in the world into holy parchment.
But there is no need for him to do so.
Everything in the world is connected to everything else.
Every detail is intermeshed with other details.
Every action has an ongoing ripple effect.
When a person changes just one particular, he improves the entire world.
The purpose of a mitzvah is ultimately to elevate the entire world, to completely change the essential meaning of existence in general.”
From Understanding the Tanya, p.155-156, by Rabbi Adin Steinsaltz