This is the only new markup introduced in Creole. Few wikis use angle brackets (<<<>>>). This element was mainly developed due to MediaWiki's extensive use of special characters making it practically impossible to implement Creole in Edit Creole Mode.
The placeholder element is generated by the engine, rather than written by the user. Therefore it seems better to reserve the easier to write two angle brackets for more frequently used markup like the GenericExtensionElementProposal and use three angle brackets for placeholder.
It seems that the notion of using three characters for block markup (see HintsOnExtending) fits better to the extension (plugin) element, but to follow to the goals of fast to type and readable markup it is better to reserve easier to type markup for more frequently used elements, leaving the placeholder with three instead of two angle brackets (see also BlockMarkupNotionCriticism).
[#1] extensions like plugins/macros might have code in the extension body that spans several lines
See also Placeholder.