The question is why a prosecutor would go through the motions of asking a warden to notify him of the availability of a prisoner that he never intends to take into custody. The first answer is that it is common practice for many prosecutors to automatically file a detainer upon learning that an accused is imprisoned elsewhere. This decision is made without any regard to their eventual decision to prosecute. But the more basic answer, and the reason why this practice of automatic filing of detainers has developed, lies in the effects a detainer has upon the prisoner.
Donald E. Shelton,
Unconstitutional Uncertainty: A Study of the Use of Detainers,
U. Mich. J. L. Reform
Available at: https://repository.law.umich.edu/mjlr/vol1/iss1/9