There is much debate about the competency of the ratings agencies, e.g. the sub prime mortgage saga, but their relevance is unquestionable.
They matter because there are thousands of pension funds (and others) around the world with trillions of $/€/£ to invest and for safety’s sake, the vast majority of them will have strict rules about the quality of investments they are allowed to make. Often resticted to AAA.
And, of course, it is the agencies who determine the quality of those investments.
The fund managers can then breathe a sigh of relief in that if someone’s bond or whatever does go belly up they can point at the agencies and their own job is still safe.
That’s why the Euro politicians hate the agencies so much, because they do matter, and would like to find a way to regulate them. Argentina would be happy about that too:-)