How does the sitra achra benefit from this spiritual energy?

As it is brought in Kabbalah, “For during all those [130] years that Adam kept away from his wife [Chava, to ‘know her’], unclean spirits came and conceived from him, and bore offspring, which are called ‘plagues of the offspring of Adam’ (Zohar I:169b, Vayishlach). The ‘unclean spirits’, known as sexual shedim (demons), are called Lilim y”s, and have origen in the feminine shed Lilit y”s. These are “impregnated” (see Avot d’Rebi Natan 37:3) from the spiritual energy of semen (spilled in vain) which was ‘not’ used in kedusha (and in this manner, impurify man) and give birth to the ‘plagues’ – another class of shedim – which flit about in the world ‘filling the atmosphere’ together with the other hordes, such as the mekatrigim (see Zohar I:190a, Vayeshev; III:196b, Balak; R' Chayim Palachi zt”l, Kaf HaChayim 240). Moreover, this progeny of shedim is very “special”, being that they are hybrid creatures of demoniac-souls (i.e., human neshamot that have no guf to be connected to, plus the Lilim y”s), which steal and so ‘starve these poor souls of their great vital energy’ (see Bamidbar Rabah 16:25) since each neshama that is brought down from its source above receive spiritual sustenance. 

Now, by Divine law, this plagues are as much considered the person’s children – for he was responsible for their fathering – as are their own children of flesh and blood in the world! These demoniac-souls then hunt their ‘causers of suffering’ (not the Lilim y”s, but their human parents that initiated all these sufferings for them), staying close to them and seeking to siphon their spiritual energy. And as it is written, “When a man goes to the side of the Left and walks in impurity, he draws to himself all kinds of impure spirits, and an unclean spirit clings to him and refuses to leave him, since these spirits cling only to those that cling to them first” (Zohar I:55a, Bereshit). This connection causes man great mental confusion and the machshavot zarot in his life, and vengeance from these plagues on the afterlife! Human beings are forbidden to father these “children”, for they profane the world and man, and seek to dominate humanity. This is contrary to G-d’s Law, which affirms: “Be fruitful, and multiply… and have dominion over the birds of the air” (Bereshit 1:28), to wit, all celestial creatures, for only “man was created in His image” (ibid. 27). It is good that man remembers in the image of Who he was created, so he can be one who is happy walking the straight path, “For the upright shall dwell in the land, and the innocent shall remain in it” (Mishlei 2:21), amen.