Gandalf and Theoden are the victors in this war and are now going to the
"castle" of Saruman, the loser in the battle, to settle terms. This is
part of the traditions of warfare, and they need to gain his surrender,
support, or at least spell out the conditions of his loss.
Saruman, being the smarmy orator that he is, will not relent and believes that
he may still have enough magic left in him to use the voice to good
effect. Certainly the weaker-willed men don't stand firm. Yet
Theoden has been hardened by his experience, and Gandalf can now use the voice
on Saruman (as he does when he commands him to return), so the power has
shifted.
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"All wizards should have a hobbit or two in their care..."
