The ability to move the seat back up and down is a commodity feature that is not very common in many chairs; however, it is a feature that will be found in multi-function task chairs quite often. For the most part, a ratchet back will replace an adjustable lumbar support, since it will usually have a passive lumbar support. A ratchet back can have its uses, though; a chair that is used by different people might need this feature, being as each user can easily and quickly customize the feel of the chair.
Final Word: A benefit. An S contoured back or an adjustable lumbar are needed for proper back ergonomics. Adding a ratchet back to those two is a bonus, but not necessary. All else equal, lumbar support will give you more ergonomic benefits than a ratchet chair back, but for 24/7 chairs, for example, a ratchet back, which allows for customization of the sit, is crucial.