For us, the main security value of this treaty is non-proliferation. On South Asia, both India and Pakistan have pledged at various times to sign. The CTBT there can help contain a deadly nuclear arms race between countries that aren't constrained by the nonproliferation treaty. If we get North Korea under the test ban they'll be less able to exploit their ballistic missile capabilities against us. The same is true of Iran, and they have signed the Comprehensive Test Ban Treaty.
Nonproliferation is an urgent national priority.
The United States is the leader of the global effort against the spread of weapons of mass destruction. A world in which the rogues have nuclear weapons would be a world of peril for all Americans. The Test Ban Treaty is not a silver bullet, but it's another valuable tool. Nonproliferation is hard, uphill work. The American people should not expect good results if the Senate denies us the means.