It's not so much that you buy them back at the same or a lower price, but that you are using the tax loss from the sale to offset a gain from other sales you have made or expect to make. You then repurchase the same shares at a similar price shortly after the sale. So you have effectively reduced a realised or expected taxable gain that would be payable, but your financial exposure is very much the same as if you had not made the sale and repurchase.
Check this out for further info on what is and isn't a wash sale. Some transactions which are deemed a wash sale are of no interest to the ATO (e.g. where the sale results in a gain).
