Product Interface is complementary to WAVE linking and improves the effectiveness of the interpart workflow. There are a number of advantages to using the Product Interface in the WAVE linking process. I will just list a couple to start the conversation going.
First one is that you can define an object (say a solid) as a Product Interface, basically you are giving it a name identifier. In Customer Defaults you can create an emphasis preference for selecting objects in WAVE link commands so that only objects can be selected for WAVE linking if they are a Product Interface. This gives control to the author of data of what can be linked and reduces the chance of someone else selecting the wrong geometry.
Another advantage is that your can change what object in the source file of an interpart relationship is the product interface, thus being able to swap geometry source side rather than target side of an associative copy.
There are couple of knowledge base articles about the 'Product Interface', a good start to know about it.
The short story is that product interface is used to prepare for linking.
My source part may have multitudes of features and objects but only several may be interesting to reference from outside the source. An interface provides separation of the private detail from the objects available for public access. I declare a source object as an interface to facilitate ease of access, or to restrict access if I need that level of control.
In the image below bodies A, B, and C are declared as interface objects.A rule grants level of access.
In a site part I use the ‘WAVE Interface Linker’ command to select from a list of interface objects in a source part. (I trust your object will be named more meaningfully than my simple example.)
The ‘WAVE Geometry Linker’ command also provides this list of objects.
The source object is linked into the site via the interface. The interface provides a level of indirection between the source object and the linked feature in the site. I can modify the source object and update the linked feature in the site via the interface. I can edit the interface to reference a different object and propagate the change to whatever references the interface. Numerous sites may reference an interface.
Interface delay and update was added in NX 11.0.1.