The goal of puppy socialization is to create positive associations to everything the dog will encounter throughout it’s life. Small dogs rarely get a chance to safely interact with large breeds and can become scared of large dogs which results in unwanted behaviors. Learning to play side by side with a large dog is a great way to allow positive interaction without scaring the puppy.
Parallel play is also a great way to have fun around dogs while keeping the puppy concentrated on human interaction rather than building a dog magnetized dog. When dogs figure out that other dogs are much more fun than humans, you can have a hard time calling them away form play or calming them down after play.