Component Interface encapsulate data and business process and provide an interface for external(Java,C++,Excel) or internal(PeopleCode) applications.
It helps when an application need to interact with your component,you dont have to write all validation check or component peoplecode, Component Interface
automatically executes the peoplecode.
If your application is going to insert one row and there is a peoplecode written on rowinsert and SavePreChange
Component Interface will execute the events.