Partner Marketing is what shows to Partners in Partner Center > Marketplace > Discover Products. While this material is set the same as End User Marketing, it differs in that it is designed to provide Partner Center users with the material they need to understand and sell the product.
Should you wish to use this optional page, you will need to toggle it on under Vendor Center > App Name > App Information > Partner Marketing.
You may wish to use this to provide sales collateral to Partner Center admins, or to provide a sales pitch on why they should offer your app.
For information on setting End User Marketing, see Creating a Basic App.