View As - This is a "filter" that changes your view to show you what another user type would see. It is useful for a quick glance but not full on testing of a workflow.
Impersonate - switches you to being that user. Much more effective for testing but also allows you to submit assignments / take quizzes etc. as that user so must be used carefully. D2L note that some institutions have disabled this for GDPR concerns or created dummy student account for impersonating.