There are a lot of ways to use the belt and lumbar pad.
The belt can be used in 3 different positions when used articulating. The lumbar can be used as an add on when articulating. In addition the belt can captured and used with a pad which is much more "normal" like most other packs. If the belt is captured, lumbar is almost required. If the belt is floating / articulating it depends.
For me, I like a higher ride. So I like the grommet wrapped . In this setting I need the Lumbar pad because there is less articulation. If I use the standard grommet setting and articulation, I really do not need the pad, but it helps the belt perform better at higher weights.
In essence the lumbar pad helps the belt function at it's best. Many people do not need it and the improvement will be slight. For some body types it is needed. Some people, like myself, just prefer a ride that works better with the lumbar pad