No, it's not imperative, but do I feel they do better with it , yes!
I give my puppies some at about 3 days old and then again just before I start them on the Goat's Milk formula with some in the the first meal of the day. I continue this until I have them on smashed kibble and formula. After that I just dose them after they have been wormed.
I should have clarified that birth, worming and weaning can be stressful. I also give probiotics within 12 hours of whelping, then as needed based on the quality of their stool.