For example, if your employer pays for you to have a subscription to a learning website, you can offer these certificates as evidence of using it.
When quizzes are available, they can be private to you or shared with an administrator or instructor if your login is part of a business account or associated with a formal class or tutoring (in the case of Khan Academy). They help you track how well you've retained new information. Some sites offer certificates upon completion. They are not widely accepted by other institutions, but they may be helpful to you in some cases. For example, if your employer pays for you to have a subscription to a learning website, you can offer these certificates as evidence of using it.