A primary advantage of using an IIFE is that it creates a private scope for variables. Variables declared inside an IIFE are not accessible from outside the function, which helps avoid polluting the global namespace and prevents naming conflicts. This encapsulation is a key concept in modular programming and was widely used before JavaScript had built-in modules. IIFEs create a closure that protects variables from being accessed or modified unintentionally by other scripts, improving code organization and reducing the risk of bugs caused by variable name collisions.