Because you will learn a lot about the case you didn't know, stuff that isn't in medical records or may not have been revealed in client interviews no matter how assiduous the preparation. You will learn anecdotes that can be very helpful in your case. Sometimes these "grains of truth" or "indicia of reliability" can be quite telling. For example, when a client says he or she had to stop working because they couldn't pull their oxygen tank next to a blast furnace and therefore couldn't stop their cluster headaches, it screams disability. This is especially true when the client had previously stabbed himself or herself in the leg with a syringe to stop the headaches so he or she could work but developed an immunity to the medicine in the syringes. This information came out in a car ride on the way to the hearing. Powerful stuff.
Most importantly, though, you make sure they make it to the hearing. They can't win if they aren't there.