Turning on Markdown enables Pensieve to display your emails in HTML using the simple formatting you specify. This works particularly well if you send yourself lists using Pensieve.
See the Markdown Guide and Markdown Cheatsheet for an overview of what’s possible.
Your choice to use Markdown Formatting in Pensieve also applies in Pensieve Share.
Pensieve Share + Markdown works great with Bear, which uses markdown to format documents. You can share documents from Bear to yourself using Pensieve, and the document content will appear nicely formatted when emailed to yourself.
Other resources: