I think that most of the time, it's mostly a way of claiming responsability for an action (or multiple actions) before the media or authoritarian organizations spread some shit about what happened. Especially when some direct action take place in a big gathering, a big protest or a "social movement".
At least, the goal might be to counter-strike the propaganda against some direct action (that generally consist in smashing some stuff, like property of companies, banks or other capitalists, or some parts of state institution like the police, courts or prisons). Generally, these actions are also on a low level of conflictuality or violence. Like it didn't involve to totally destroy an entire building, something like that, or to use explosive material, to hurt anybody or even to terrorize anyone. In fact, most of the time this kind of action is supposed to be mainly "symbolic" (even, to me, it's always a way to attack the system of domination and oppression, and where the goal should never be to "make a symbol" which reduces any action to a spectacle).
This question of reducing this kind of action to something spectacular, which the media contributes to, are also a great question in the anarchist spaces and movements all around the world. And if the goal of a communicate should be to explain the action(s), many anarchists supporte the idea that it shouldn't be a way to give some consistancy to something you say : but only to join the gesture to the speach, and vise versa.
Communicates seems also important to many anarchists because they think that if sometimes "action speak louder than words", all actions don't "speak" for what it have been done, and that you may want to explain it or complete it with your words.
As long as it seems necessary and appropriate to you : like not to comment the fact that you set fire on a dustbin, and then produce some unbearable revolutionary gossip.
And so I think that I guess what you question underlies : is it always usefull and does it always make sens to use this formula "smash + communicates" ?
Regarding to what I wrote just before, my answer is no : of course.
Some anarchists from barcelona wrote this funny text criticizing the anarchist-nihilists of the FAI who just gone to far with the "communique-mania". You can read it here : http://anarchistnews.org/content/communiqu%C3%A9-anarchist-actions-barcelona-and-response-nihilist-comrades
There is also a very good italian insurrectionnary anarchist text written about this, called "About some old issues of actuality among the anarchists, but not only". Unfortunatly, it wasn't translated to english yet. But if you read italian, german, dutch or french, you can find it here : http://translationcollective.wordpress.com/2010/02/04/su-alcune-vecchie-questioni-dattualita-fra-gli-anarchici-e-non-solo/