For about the same reason I acquired a digital scale. Say if a package I am shipping is going to be 8.1oz with proper packing, I just give it more wraps until it gets close to 9 oz but still stay below 9oz. I ship most times in bubble mailers and then a bubble wrap and then a baggie with parts.
BTW, since there is such a thing called a filler (say $0.99 paper clip to make total order >$50 for free shipping or something), did anyone receive a box only with a filler in it? I did once. Waste of resources.