To install a mod, look at the appropriate section in the FAQ (
link).
The CBM Mod changes numerous things, ranging from costs/abilities of recruitables to level/cost/power of spells, the effects of scales, and the costs/abilities of pretenders. It also includes the Worthy Heroes mod (a mod the adds new and better heroes to nations that don't have any/enough).
While they all do work together to diversify strategies, the option to install the parts seperately is for the convenience of those who like some, but not all, of the mod. For example, if you like the pretender and spell changes but you don't like the changes to recruits (which can be odd at times), the capability is there without too much work or modding knowledge.