Within View Renderings and Controller Renderings (at least) there is a shared field within the Data section called Placeholder. What exactly is that field used for?
It is a default, used when no placeholder key is given upon inserting the rendering onto the page. This can happen both when editing via Content Editor/Presentation Details or as a result of a Conditional Rendering action.
The Placeholder Rendering Property
The Placeholder rendering property in the Data section of a rendering definition item specifies the default placeholder key for the rendering. Sitecore uses the Placeholder rendering property as the placeholder key if a user does not specify a placeholder key in layout details or if a developer does not specify a placeholder key when adding the rendering to layout details at runtime using conditional rendering.
Source: Presentation Component Reference