Because you will have waited for her to come in season, counted the days and taken her to the stud dog on the correct day, then counted the number of days from the successful matings, and taken her for a scan if she is showing no signs, just to see if there are any, and so you will know whether she has reabsorbed the puppies, and to help you decide whether she might need a caesarian section if she goes past the due date, and to decide whether she should be mated again in future.
Or not.