George W. Bush is still very unpopular and gets a high degree of blame for the economy, despite leaving office almost four years ago. The latest ABC/WaPo poll shows 54% blame President Bush for the country's economic woes, compared to 32% for President Obama. Republicans, wisely, have been trying for some time to say Bush is old news, an excuse, etc.
Casting the debate over where the country stands compared to four years ago also plays into the Obama camp's push to say Romney is Bush redux. Again, good for Obama, bad for Romney. And lest we forget, Romney is (rightly) closely identified with Wall Street, ground zero of the calamity.
Four years ago isn't a perfect question for Obama. But then it's not a perfect time for President Obama to have to run for reelection, is it? But it does set up all the key arguments the Obama campaign has tried to make. And Romney as Bush is something like the Holy Grail of campaign awesome to the folks running Obama's campaign.