Normally, when you are working on a test/staging/development site, you don't want to have the mail option activated, so that you can avoid potential faulty email sending and/or spamming. For such cases, you usually disable the email sending functionality, by navigating to: Administer CiviCRM -> System Settings -> Outbound Email (SMTP/Sendmail) and switch to 'Disabled Outbound Email'.
However, there are some pitfalls by doing that:
If we have a function that tries to send an email, regardless of the settings you've provided, that function will crash (WSOD) and exit. A less catastrophic alternative is that you simply want to see how the emails will look like (or if your function is sending any emails) without actually sending them.
That's where this extension kicks in: By setting the email sending option to 'Redirect to Database' and by using this extension, you can now see how the emails look like, without crippling the functionality of CiviCRM.