These are the forms as developed by the late President & Founder of the World Hapkido Federation, MYUNG Kwang Sik Kwan Jang Nim. So, at least in the WHF we know how/when they were created and incorporated.
In some arts, they created forms later in the system's history because as it spread and more people became aware of martial arts they wanted forms/katas to compete in tournaments and also it was more material to learn to spread out the belts. For example, Prof. Chow didn't have forms in his art and then later added them because that was what people wanted. Ed Parker, his student, didn't originally teach forms but later added them because that was what people wanted/expected and even created later forms specifically for Kenpo students to compete with in tournaments (Long 4 for example). It could be this reason, or the person wanted to string together certain techniques and patterns for a student to be able to practice them in a more fluid manner and easier to remember.