1) Not showing you hand allows you the aura of "objectivity" and the opportunity to say you "listened carefully to both sides before my vote." Senators love this self-stroking of their own ego.
2) Some of them want something. A deal. A sweetener. In the case of Flake, it might mean that he wants a guarantee that he won't be prosecuted for his crimes (and yes, Flake is a criminal).
The evil radiates from his face.