What's there to gain?

Well, we know that the Dark Eldar keep their psychic potential in check with drugs, etc...

(In character)
Perhaps with the growth of an implanted gland naturally synthesizing these chemicals, the effects can be further strengthened? Or perhaps modified as a natural defense mechanism to Warp phenomena, similar to the so-called "fight or flight" response, or how octopods cloud their escape? So many possibilities...

(In character) I have heard of Haemonculi infiltration into other covens and the combat drugs they administered calcified the brains of the kabalite warriors, they became extremely open to suggestion Some of the implants or even said to be capable of puppeteering the subjects. I don't believe in trust, only common interest. Finding that in the dark city is 99.9%
of striking a pact