
For me there are two main reasons for scanning. I can let people on my waiting list know if there is a chance of a puppy, or if not in whelp get them onto aniohter breeders waiting list in a timely fashion, so it is a service to my puppy enquirers.
The second important reason in a breed that puts on weight easily and looses it not so easily is that I can keep their food intake down until I know they are in whelp for sure. when pregnancy is confirmed I up the quality of the food, but do not increase volume until the sixth week, slowly building up to about half as much again as normal.
A third reason for thiose with show dogs is of course that you are not wasting money on speculative entering of shows, and if she isn't in whelp won't miss entries closing for show, which can close up to two months ahead.
I have never relied on a scan to determine number of pups, but have used it post partum to check if there were any more to come.