The development level work like this.
For example. When there is a new warrior David in the area
The base attributes of David are
When the development level is 7.
There is a chance that one of the attribute add 7%.
The development level affect the quality of items too.
For example for a bow with 20% remote.
There is a chance to improve the bow to 27%.
The chance to improve is between 20% and 80% for different person or items.