Generally speaking, a prohibited action on Shabbat is only forbidden if it is done for a constructive purpose.
We learned in the Mishna: And anyone who performs labors destructively on Shabbat is exempt.
Rabbi Abbahu taught this baraita before Rabbi Yoḥanan: Anyone who performs labors destructively on Shabbat is exempt, except for one who inflicts a wound or kindles a fire.
Rabbi Yoḥanan said to him: Go teach that outside. This baraita is not fit for discussion in the study hall. The opinion that deems one liable for inflicting a wound or kindling a fire on Shabbat is not an accepted teaching and should be ignored. And if you want to say that it is a legitimate teaching, one who inflicts a wound would only be liable in a case where he needed the blood to give to his dog, and one who kindles a fire would only be liable in a case where he needs its ashes.
This brief passage is pivotal in the comprehensive discussion among the commentaries and legal authorities with regard to the law of melakha she-ainah tzrikha le-gufa – primary categories of labor not needed for their own sake, i.e., their purpose in the Tabernacle. For example, extinguishing a fire to conserve fuel, rather than to produce coals (the purpose for which “extinguishing” was performed in the Sanctuary) is considered melakha that is not necessary for its own sake. According to other opinions, melakha that is not necessary for its own sake means work performed for a “negative” purpose; in the previous case, for example, extinguishing a fire to avoid using the fuel, rather than to create something new (i.e., coals). There is a controversy among the tannaim as well as among later halakhic authorities as to whether melakha that is not necessary for its own sake is prohibited by Torah law or only by rabbinic injunction.
The Me’iri summarizes the issue as follows:
If one performs a primary category of labor in its standard, constructive manner it is an act needed for its own sake. An example of this would be the primary category of winnowing.
Even though the main goal is to remove the chaff, it is still considered a primary category of labor needed for its own sake.