I'm with those who've put forth the idea that it was about dealing a symbolic and psychological blow more than it was about dealing lots of actual damage. Plus, don't forget, it's reasonable to assume that at any given time, there is a fairly high concentration of high-level Starfleet Admirals within the buildings that were hit; no doubt several of them were killed. Not that it would be SUCH a blow to their strategic capabilities; Starfleet of course would have plenty of other admirals around, but coupled with the morale-crushing aspect, and the fact that some of the survivors of the attack would probably have a hard time getting "back to work" after going through what they did, and it all adds up.
We also are never told just how many Breen ships take part in the mission. It stands to reason that it wasn't a HUGE number; getting a massive fleet all the way to Earth unopposed seems unlikely. I'd wager it was just enough ships to quickly inflict the kind of damage we saw, and that some form of trickery (anything from an informant in the Federation government to sabotage of local detection systems) played a role in allowing the op to succeed.
Cant we just shorten the question to 'Breen- foolish'? Nothing about the Breen made any sense, a real low point of that pretty awful last season. We can make excuses about how and why they did it, but lets just face the truth. The writers didnt know how to up the ante in any other way! This is, of course, the same episode that gave us the totally pointless destruction of the 'Defiant'.
You are entitled to your opinions of season seven and the Breen, of course; I happen to disagree.
But I've never understood this whole "Let's face facts, people!" attitude that I see often in posts - such as yours - that are, in fact, dealing entirely in subjective matters. There is no "truth" to face. I can't speak for anyone else, but I'm not "making excuses". I
liked the Breen, I thought they were interesting (if a bit underdeveloped), and I love season 7.
When I watched the episode I always thought a better target would have been the ship building yards orbiting Mars but for pure shock value attacking Earth raises the stakes.
If one assumes (as I tend to) that something along the lines of what I put at the top of my post is true, that the Breen didn't have a large strike force since too many ships would make it impossible to get them into position undetected, attacking something like a shipyard may not have been practical. Doing enough damage to such a facility to make any kind of real difference to Starfleet's tactical capability may have simply taken more firepower than they could have on scene, whereas with an attack on Starfleet Command, the goal (which, again, isn't physical damage, but morale damage) is achieved simply by having enough firepower just to blow up a few buildings.
What bothers me with Trek in general is the Federation has all of these super weapons and time travel. I think long before the Federation is close to defeat someone would go back in time and just block the wormhole.
Except they
don't have those things. At least, that's what the writers want us to believe every time they threaten the UFP.
Borg invasion? Go back in time and warn Starfleet about the Borg decades in advance.
Sisko went back in time and changed history by getting Gabriel Bell killed? Figure out what year he and the others are trapped in, take the
Defiant for a spin around the sun (a la TVH), and beam em' up before any damage can be done.
Dominion winning the war? As you say, go back in time to any number of points to help alter that outcome.
Really, the creators of Trek have introduced FAR too many ways to time travel over the years; these must simply be ignored in order to make some stories work. It's not that different from how certain abilities or technology that should make a problem easy to solve are also conveniently ignored in an ep to create tension. Mind you, I'm not letting them off the hook for THAT, either; it happens far more often than it should.
What "super weapons" are you referring to, though?
^
That's when the temporal police introduced in VOY come in & say, "that wasn't supposed to happen."
I never liked them.

I didn't like their depiction of the 29th century, and the idea of temporal police from the future that actually zip around time and space doing the Quantum Leap thing is just silly.