In a court of law, it is important for a prosecutor to have a motive (reason) for a defendant to commit a crime (particularly a murder). When a prosecutor is presenting evidence, they must prove, beyond reasonable doubt, that a defendant is guilty. Part of doing so requires showing that the defendant has a motive for committing the murder. For example, in "Trifles," without the canary, the county attorney does not have a motive. This is why the women ultimately decide to NOT let the men know about the canary. Of course, this was not the ONLY motive for killing her husband (it was years of abuse), but it was the last straw for Mrs. Wright to kill her husband.