The Sprout Studio Assistant is geared towards reminders and notifications and was made for you to keep your clients in the loop. Sprout Assistant emails are designed to be a pretty, branded studio notification to your clients; they're less of a "Hey! How are you? How are the kids? I wanted to touch base and say XYZ."
Sprout Assistant is also used throughout Sprout Studio where there aren't any workflow management tools, and probably never will be because it would become impossible to maintain. For example, Sprout Assistant integrates and works with dates, galleries, design proofs, and so on.