What is the purpose and use of evidence of a first complaint?

The purpose and use to be made of the evidence of a first complaint was set out in Regina v. Lillyman, [1896] 2 QB 167, when Hawkins J. said at p. 177: “The evidence is admissible only upon the ground that it was a complaint of that which is charged against the prisoner, and can be legitimately used only for the purpose of enabling the jury to judge for themselves whether the conduct of the woman was consistent with her testimony on oath given in the witness-box negativing her consent, and affirming that the acts complained of were against her will, and in accordance with the conduct they would expect in a truthful woman under the circumstances detailed by her. The jury, and they only, are the persons to be satisfied whether the woman’s conduct was so consistent or not. Without proof of her condition, demeanour, and verbal expressions, all of which are of vital importance in the consideration of that question, how is it possible for them satisfactorily to determine it?”