The link between the laws of Shabbat and the Mishkan not only defines the 39 Melachot but also determines the conditions for liability. One of these conditions is intent. The other is purpose.
The melachah must be performed for a similar purpose as the act performed in the Mishkan. Accordingly, one might, intentionally, perform the same act performed in the Mishkan and yet be exempt from biblical liability if it did not have a similar purpose.
For example, digging (a derivative of plowing), was performed in the Mishkan for the use of the hole itself, in which tent pegs were sunk. Therefore, one who digs for earth and has no use for the hole has not performed a melachah in the Torah sense of the term, a melachah de’oreita.
Similarly, extinguishing a fire, a primary or av melachah, was performed in the Mishkan to produce glowing embers needed to smelt metal. Therefore, one who turns off the light in order to sleep, or to save electricity, has not performed a melachah de’oreita. Such an act is known as a melachah she’eina tzericha l’gufa. Although biblically exempt from liability once performed, a melachah she’eina tzericha l’gufa is rabbinically prohibited, a melachah derabbanan and should not be performed in the first place.
What is the difference between a melachah de’oreita and melachah derabbanan if both are prohibited? The answer is that generally there is more room for leniency in melachot derabbanan. For example, melachot derabbanan may, mostly, be performed during twilight, bein hashmashot, on Erev Shabbat; they may, with certain restrictions, be performed by a Jew on Shabbat to alleviate substantial pain; they may, in certain situations, be performed by a Jew on Shabbat in order to avert a substantial financial loss; they may, in certain circumstances, be performed for a Jew on Shabbat by a non-Jew; and they are not themselves subject to protective legislation.
Because the melachah she’eina tzericha l’gufa is closest to a melachah de’oreita, in that it only lacks the element of common purpose and because there is a dispute with regard to its definition, the rabbis are less lenient with it than with other melachot derabbanan. Accordingly, it enjoys some but not all of the flexibility described. For example, a melachah she’eina tzericha l’gufa may not be performed bein hashmashot. Such a melachah may, however, for the most part be performed by a Jew on Shabbat for the sick, even the not dangerously sick; and in certain situations may be performed for a Jew on Shabbat by a non- Jew.
Based on these principles, Rabbi Shlomo Zalman Braun, in his work Sha’arim Metzuyanim B’Halacha, writes that sparks ignited by plugging in or out of electricity is akin to a melachah she’eina tzericha l’gufa, in that it is a psik reishe delo neecha lei, which means an inevitable melachah arising from a permitted act that is of no use to its performer.
Accordingly, to avoid substantial financial loss one may ask a non-Jew to reconnect a well-stocked freezer that became disconnected from its electricity on Shabbat. Similarly, one may ask a non-Jew to turn off an appliance which, if left running all through Shabbat, would overheat and burn out.