I have felt that the purpose of the knife was to rattle Knox and Sollecito's cages a little. The idea was to get K + S's reaction. If they denied it was the knife, of course they would be guilty. It was sort of a hail mary pass made in desperation, because the evidence was coming back and it was not pointing to Amanda and Raffaele. Magnini was hoping that in their reaction they would say something to implicate themselves.
Supporting this idea is the fact that what they did say has been used to implicate them, even though it is not implicative. Raffaele's attempt to come to terms with the inexplicable, by incorrectly thinking maybe Meredith could have cut herself, made in a phone conversation to his father, and Amanda's statement that she is worried about her situation due to the claim about the knife have been used to insinuate guilt by the court, and of course the guilters.
The prosecution intended to use A+K's reaction to the news about the knife against them before releasing the information. It was a planned trap. The trap did not work, but the prosecution took what it could from the remarks that were made, and tried to spin them to look like guilt.