Why do menu items blink twice when selected?
It is simply a visual cue that you have selected that menu item.
It is described in Apple's &quot;OS X Human Interface Guidelines&quot; under Menu Behavior (page 134):

When the user chooses a menu item, it blinks briefly to confirm the
user’s choice and then performs the action.

(Note that this document has been retired and is no longer available for download, but a copy is available at https://issuu.com/hyungjoonjun/docs/osxhiguidelines)
OS X provides other visual cues: when you press a key shortcut, for example ⌘+C, the menu bar menu where that key shortcut is listed, in our example Edit:

is highlighted for a fraction of a second.