Step by Step
There is a mitzvah to be holy. To be special. To be separate and distinct from others. One way to do this is by separating oneself from permitted unnecessary physical pleasures. All the mitzvahs in Parshas Kedoshim lead to holiness, even the peah -- the corner of the field, approximately 1/50 of the whole, that is left for the poor. How much could a little bit make a difference? But it does.
A famous middle-age commentary says you should not clean your plate when eating. And leaving leftovers is not a violation of the prohibition not to waste because you are doing something constructive, you are subduing your cravings. You are teaching yourself to resist something desirable. This skill can be transferred to the yetzer hara, the desire to do the wrong thing.
"You should be holy for I, HaShem, your G-d, am holy." Leviticus 19:2, from this week's double Torah portion, Achrei Mot & Kedoshim, Leviticus 16:1-20:27