What makes a hydraulic torque wrench work is that it generates torque using only hydraulic means, it is self ratcheting, and it must have an accurate way to measure the amount of torque applied to the fastener. A holding pawl design is popular amongst most manufacturers to lock the wrench in position, but each have some claim that there design is superior. I say whichever one works, works. 

These wrenches typically have an accuracy rating of +/- 3%, with a high repeatability rate. This makes them suitable for large bolts that need a high degree of accuracy. The biggest advantage of a hydraulic wrench doesn't have anything to do with performance, but with those who use them - these torque wrenches are significantly quieter than similar output pneumatic impact wrenches. That is a benefit that you can't measure in strength, repeatability, or work output increases - it is about the health of those who use it.