When you flag an email in Outlook, it is because you want to follow up on this email. You can also choose when you want to get reminded.
The email will then appear as a task in Flagged email in To Do.
For flagged email tasks, you can add details, due dates, steps and attachments to it, just like any other task.
Checking the To Do box removes the flag in Outlook, and checking the flag in Outlook completes the associated to-do task.
If you're using Outlook on the web (OWA), you can also right-click an email in the message list, hover over Advanced Actions, and then click Create Task to add it to your To Do.