I don't see OU-Parve often, but when I do it is usually (possibly always; I don't remember a time when this was not the case) on an item which by default would likely be assumed to have some status other than parve.
The example that comes to mind is soy ice cream. Many people would assume ice cream is dairy and even if they saw an OU without the "D" for dairy, it may not even occur to them that the product is not dairy (not because they think the OU would neglect to label something dairy, but simply because the thought of ice cream being parve didn't occur to them and they weren't paying that much attention to the [lack of] specific designation on the ice cream container). In this case, the parve designation does two things:
- It brings the parve status to the attention of people who might otherwise not have thought of it.
- Similarly, it affirms the parve status to people who may have had doubts.