The forward launcher, the old “single-armed bandit” was primarily for a surface-to-air missile, the SM-2. The Perry-class frigates also fired Harpoons from them.
Nowadays, Harpoons are fired from canisters loaded onto racks located somewhere topside, and the SM-2 is fired from a Vertical Launch System.
This permits a far greater rate of fire, and allows the simultaneous use of the SM-2 and Harpoon weapons systems.