Why? Because of her cash connections. She helped raise a whopping $65 million for NYC schools and hosted pricey fundraisers for Obama through the election. [Here's one of her invites.] Sadly, the ability to raise money is an important qualifier for an aspiring political candidate.
If she goes to the Senate the Democratic party will have easy access to the Kennedy gravy train. That includes Governor Paterson, who will be running for re-election in 2010. If he gives Kennedy the nod, she'll owe him one.
I'll give a shout-out to any commenter who can name a policy position she's publicly taken in her lifetime.