No one likes it when bad things happen to good people. Sending a sympathy card, even when you don't know exactly what to say, shows that you care. The most important thing to say is that you're there for your friend or family member. Even when you can't be physically present, being available for a phone call can mean the world to a grieving person.
We offer a few different cards based on our own experience with loss and grief. Sometimes, you really don't know what to say, which is where our "I have no words but I'm here" comes in handy. It's also perfectly acceptable to simply say "I'm thinking of you," so we've included those cards in our sympathy card collection as well.