We should take a view as if we were in the position of a dictator, able to give rules to all of society. What rules would we wish to impose, in order to produce a society that we judge best? Naturally, this judgment of "best" is subjective, but there are similarities between what different people want society to be like. Most people would like to see the society prosper - become more vibrant and efficient. Most people also want to improve some measure of utility or happiness for society.
In order to achieve this vision, it is useful if people are rewarded for actions that help society (in the judgment of the supposed dictator), and punished for actions that harm society. If these rewards and punishments are in place and consistent, then people will be more motivated to help society and not harm it.
For example, burning tires in a residential neighborhood is undesirable; it creates toxic fumes and a terrible smell, which makes nearby people unhappy. From our imagined position as dictator, we don't want people to do that; it goes contrary to our vision for a healthy society. So we would like to have a rule that punishes people for doing it. The rule should not cause more harm to society than the benefit it provides.
Thus, being punished for burning tires in a residential neighborhood is a just desert.
Or more accurately: if you judge that letting people be unhappy is detrimental to your vision for society, then you should also include a punishment for burning tires in a residential neighborhood in your vision, and thus you would say that this punishment is a just desert. It's conceivable that others would have a different vision for society in which the punishment is not a just desert, but you would say it is, which is what matters to you.
Similar arguments let us say:
- It is a just desert for murderers to be punished enough to outweigh any benefit they might gain from it. This helps disincentivize people from murder.
- It is a just desert for people performing necessary community service to be paid or given honor for their actions. This helps incentivize community service.
- It is a just desert for people who expose corruption in the political machinery to be rewarded for doing so, not punished, because their actions help society as a whole and we want more of that.
Note that this concept of "desert" is different from what the laws may actually be in current society. It instead relates to what laws would, hypothetically, have to be in place to help promote your ideal vision of a society.