She had to explain why she did not enforce the ban issued by the President. Senator Cornyn asked (full transcript available here) about Ms. Yates whether that decision was inconsistent with her role to reasonably defend the actions of the administration. She answered,
It is correct that often times, but not always, the civil division of the Department of Justice will defend an action of the president or an action of congress if there is a reasonable argument to be made. But in this instance, all arguments have to be based on truth because we're the Department of Justice. We're not just a law firm, we're the Department of Justice.
Answering questions from Senator Kennedy, Yates said that any defense of the president's action would have to include the argument that the executive order had nothing to do with religion, and based on candidate Trump's past statements, that would be a difficult argument to ground in truth.
Her response on that theme reflected careful attention to the exact nature of her role, which isn't to simply defend the administration no matter what, but to that the government must always ground its arguments in truth. Truth is the truth not subject to alternative versions. To Yates, the DOJ is "not just a law firm," and a prosecutor is "not just an advocate." "While lawyers representing private parties may - indeed, must - do everything ethically permissible to advance their clients' interests, lawyers representing the government in criminal cases serve truth and justice first. The prosecutor's job isn't just to win, but to win fairly, staying well within the rules." United States v. Kojayan, 8 F.3d 1315, 1323 (9th Cir. 1993)