Success states have become synonymous with celebration, particularly in user onboarding. But the fact is that guiding a user through a complicated flow of actions with positive feedback is more than celebrating a milestone. It's expanding the first conversation that users have with your product.
In many workflows, a screen makes a demand of the user and when the user complies, the product moves on like nothing even happened. Taking a beat to verify that all is well can give the user 1) mental closure and 2) feedback that the product is listening and reacting.
Make them visual (confirmation and green go great together!) And immediate (don't wait until a user has finished filling out a form to let them know there's more password work to do.) Remember that it's a conversation (your user communicates in interactions, your product responds in success states)
A well-timed pat on the back can simply feel good to receive, especially when it's for a meaningful accomplishment. These success states are good ol' fashioned celebrations of progress.
Success states work best for achievements that are meaningful from a user's point of view—this way you're not celebrating their activity within the product, you're celebrating their taking another step towards a better version of themselves.
Save encouragement states for milestones. Empty praise usually erodes motivation instead of amplifying it. You can use success states to do more than one thing at a time: combine "encouragement" and "signpost" success states to celebrate users and steer them towards a follow-on activity.