Two completely different points:
1) Let's take a hypothetical situation. Let's say your mates house is broken into and he's beaten up and robbed. Now you can say well the doctor will help with the injuries, the police will help secure the house, the insurance will help replace his stuff. But you think as a friend what can I do, what should I do? I guess even a hug may help... but I don't hug, I'm a bit crap when it comes to emotional stuff.
2) I often think "what's the point of telling people stuff because there's nothing they can do about it". So once they do manage to tell you and you, in effect, say "sorry, there's not a lot I can do about it", well I guess it reinforces their negative beliefs, which is not good.
And in situation 1 & 2 people can say "well, you were there" and "it's good to have someone to talk to" but your inability to be of much use just makes their situation seem worse maybe?