I do the ultrasound to confirm pregnancy, like Pat I want to be able to tell my puppy buyers that there really are puppies expected. Also, it is nice to see that they are progressing normally. I don't normally do a second ultrasound but have found that both my mom's vet and mine will do them for free if we come in for something else just to satisfy their curiosity that the pregnancy is on track.
I do the x-ray to make sure that I know how many pups to expect, I want to know if I need to call the vet.
I know of one litter of Manchesters that may have been saved if the owner had done ultrasound or x-ray. All the pups were in one horn and the bitch's uterus twisted when she went into heavy labor because of the uneven distribution. If they had known before labor that the pups were so unevenly distributed they may have been able to save the pups by doing a c-section.