Weight and dimension approximation for UPS plug-in

In your doc it's mentioned that the fields for weight and dimension approximation are there for us to add percentages. I don't understand this. Am I to increase the weight or dimensions of my product by a certain percentage? Or something else? Why would I need to increase the weight or dimensions?

Yes, that's indeed the case.
For example, suppose that you sell laptops. It's important to display the weight of the laptops on the product page as it is one of the criteria for some people to buy a laptop they can easily transport.
However, when you ship the laptops, you ship them with their packaging. That results in an increase on the weight that you might want to take into account when sending the weight of the product to ups.