As i have come to understand this is only important in edge cases: For example if you have a transaction that doesn't get confirmed. Sooner or later you can try to send your iotas to a different address (maybe even of a diofferent seed) before you will try to then send the transaction to the first target address again and so you will create a new transaction even though the first one from the same address hasn't gone through.
Now there are two hashes from the same address in the tangle and an attacker could try to break the key of the sending address. Now you have kind of a race condition. If an attacker breaks the key and creates a malicious transaction with the key BEFORE one of your transactions goes through, you will most likely loose the iotas in that address.
I always assume that you use the IRI or GUI Wallet that always moves your iotas around in bundles. Maybe there are other cases in different implementations.
This is how I understood this, but I'm not 100% sure, maybe someone who is 100% can add his 10 cents.