Lock ring cylinders are great for when you want to reduce the risk of unexpected piston retractions. Imagine if you are lifting and holding a heavy structure with a set of standard cylinders and one of the seals in one of the cylinders fails. If they are standard cylinders, with no lock ring, then oil will escape from the cylinder and the piston will retract due to the weight of the structure on the piston. However, with a lock ring cylinder, the piston is threaded and has a collar which can be made up tight against the top of the cylinder body. This means that even if the cylinder was to start to leak or the system lost pressure somehow, the extended height of the cylinder would remain the same. It would not retract due to a loss of pressure.
These types of cylinders are widely used in bridge building where a construction company may lift a bridge section and then keep it in a suspended condition for extended periods of time. If they use the lock ring cylinders, then they can avoid any unexpected changes in piston height from pressure losses.