The hock is a complex joint and is analogous to our ankle joint. While it is the most common site of lameness in the hind leg of performance horses, I don't believe one should assume it to be where the problem lies whenever a horse is sore behind.
The early life problems in the hock, as in weanlings or yearlings, are usually seen in the large upper joint of the hock. When this joint is inflamed, it often becomes obviously distended with excess joint fluid that is termed a "bog spavin." A common cause for this condition in these growing horses is referred to as OCD (osteochondritis dissecans), where a fragment of bone and cartilage dislodges in the joint. These lesions can be cleaned up with arthroscopic surgery and usually have a good prognosis for future use.
As horses mature and start being worked, the more common site of problems is in the lower hock joints. These lower joints do not communicate with the large upper joint and you don't see any obvious swelling when they're affected. These lower joints are smaller with very little motion, but seem to suffer the effects of concussion and torque more than the larger hinge-type upper joint. Diagnoses of these hock problems are based on factors such as history, character of lameness, response to flexion tests, local anesthesia and radiographs. Problems in this area of the hock often respond well to injection with anti-inflammatory medications.
One of the problems I've observed with this syndrome is that symptoms can be rather subtle or vague. The horse isn't really lame, but its performance has been negatively impacted. Because the problem is so common and response to treatment has been historically positive, owners request getting the hocks injected on an empirical basis. I believe you should take this time to rule out other potential problems before taking action.
I also believe a lot of horses have gone through hock problems without being noticed as having been sore. I base that on seeing a lot of older horses with obvious bone spavins that have had no history of lameness. The end result with these lower hock joint problems is fusion of the joint with bone, which is usually a pain-free condition.