After an airliner touches down, why do wing spoilers help the wheel brakes slow it down?

Answer: They cut the wing's lift so more weight presses the tires onto the runway.

They cool the brake discs so the wheel brakes can slow the plane without fading. — No. Brake heat is a real concern, but spoilers do not mainly help by cooling the brakes. They cut lift so the tires press harder on the runway while the brakes are applied. Cooling the brakes and increasing brake grip are separate jobs.

They cut the wing's lift so more weight presses the tires onto the runway. ✓ — Right. The panels on top of the wing rise and break up the smooth airflow, so the wing makes much less lift. With less lift holding the plane up, more of its weight rests on the landing gear. The tires then press harder on the runway, and the wheel brakes can slow the plane more effectively.

They push the wings downward so the tires grip the runway with extra force. — No. Spoilers do not add a downward push that makes the plane heavier. They remove some of the lift the wing was making. That shifts load onto the wheels, which is different from pressing the wings down with extra force.