The reason for inventing Combined Skills is, firstly, because their power is stronger than ordinary Composite Skills, and secondly, because it's generally not possible for one person to master various Secret Skills.
Since Secret Skills have various restrictions in cultivation, it's difficult for a single person to meet all these restrictions and learn all the Secret Skills required for Combined Skills. This led to the idea of multiple people using Combined Skills.
Combined Skills are essentially a solution for a single person being unable to cultivate certain Composite Skills; however, it was later discovered that since Combined Skills involve multiple people, the Primordial Energy is more abundant and powerful than using Composite Skills solo.
However, Combined Skills require strong teamwork; it's not something a few people can master easily, so ordinary people rarely practice them. The primary users of Combined Skills today are the military.
