Typically, when you create a LabVIEW application, you start at the top-level VI and define the inputs and outputs for the application. Then, you construct subVIs to perform the smaller tasks within the top-level VI. This modular approach is one of the strengths of LabVIEW. You can easily create complicated applications that are hierarchical in nature and reuse common elements within an application.
Using subVIs allows you to make your applications easy to understand, debug, and maintain.