Klingon Culture is not different from human culture. In our not so distant past, humans designed an ordered rule called Feudalism. The Klingon Empire is a reflection of that type of ruling order and what it might look like with futuristic technology.
The Minor House (Stronghold or Keep)
A Klingon head of a house will have the oath and fealty of many families. This doesn’t just provide armies of warriors. A house cannot function solely on the strength of warriors. This level of hierarchy is the human equivalent of wealthy, low ranking, medieval lord or knight of a “keep” or “stronghold.” This house head will also have other resources provided to him from those who have decided not to follow the Warrior’s Way. For example, there are farmers, technicians, accountants, engineers, tax/tribute collectors, and weapons makers. So there are many fields that do not require a warrior.
Some of those necessary fields Klingons despise with great passion, accounting being one of those. The Klingons recognize the importance of those areas. But the Klingons view wealth as a strong family, legendary deeds, defending honor and oaths to the death, and up holding the Empire with deeds and actions, not latinum strips. The acquisition of currency as wealth is one of those things that is desired to delegate out to other species. Which is why Quark, at least partially, was accepted by the High Council as the head of a minor house. (The House of Quark)
The Major (Great) House (Noble Estates)
This is why there are so many Klingon houses. Each house has its own unique resources. This is also why we hear stories of the importance of allying with other Minor Houses to make a “Major House” (Great Houses) or the joining of a Minor House to an already established Major House. A Major House is a noble house. The major house is comprised of many minor houses and families. The major house leader is the equivalent of “Lordship, Duke, or General” as the head.
The Head of the Major House is chosen is by birth or combat, usually both. Once a head is chosen, (which is true of all houses) that place must be earned and it must be maintained. A Klingon would never challenge the head of any house (minor or major), unless he knew he could win, prove a birthright to it, or marry into it. Taking this position is a big deal for Klingons and the Empire. A Major House is as large as a Star System and has millions of members. The influence of the wrong or right Klingon for the job can have huge ramifications and affect everything. This is seen in real human history and fictional mythical history as well, think Game of Thrones in space.
This is why the audience does not often see other species of the Empire. Those Major Houses that have entire fleets or multiple fleets, receive orders from the High Council or are patrolling their home Star Systems. This is the greatest population of Klingons because, generally speaking, every Klingon wants to be a warrior. The other species who have vowed the Warriors Way, would be less than 1%. The odds are that the audience will never see another race on board a Klingon Bird-of-Prey. But that doesn’t mean they aren’t there.
This is also why the audience rarely hears anything about the minor houses or families. The main players in the Klingon Empire will be of the noble houses. Those noble houses will be a part of the High Council. Just like in real history, we don’t generally talk about or know about the extended family of the Medici House (A human equivalent). We study the noble house as a whole.
Going back to Klingons, any relations or fealty to that major house from other minor houses are considered to be as that major house. These facts are only relevant to the members of those houses. It is a matter for Klingons. It isn’t the business of outsiders to know the details, usually. However, there are exceptions, like in the case of Curzon Dax (An outsider looked in).
Glory to the Empire, Honor to the House
It is also important to note that Klingons do not enslave or force beings into service. The bond of a Klingon is alive within him. It is his soul. A Klingon will go where his bond leads him. Sometimes, families and houses will switch alliances and they are free to do so. Klingons view non-Klingons in the same manner. A Klingon cannot control the bond of another. For Klingons it is far better to earn the respect and fealty made by one who is free to give it. So, the head of any house will take pride that he has earned and maintained the respect of those who freely follow him. It is an honor. A Klingon will give his life to protect it.
This does not mean Klingon do not conquer other races. They do all the time. But the Klingons are like the Roman Empire in this way. Klingons do not care to conquer a civilization for the purposes of spreading religion or their way of life. They do it for the glory and also for the resources. The conquered are free. As long as that civilization pays the due tribute or tax and doesn’t interfere with the Klingon provincial rulership (Just like Rome), these civilizations are able to live normally (generally speaking).
In that same mindset, the Klingons do not haphazardly conquer worlds for simple domination purposes. The conquered civilization gets in return protection from other races that would conquer for domination purposes. The Borg being a perfect example. When the Klingons conquer a race it is their obligation and their duty to protect it. The Klingons admire civilizations that fight well against them, whether Klingons win or not. It is an honor to protect that race who has earned their place in the Empire.
By that same token, this is why Klingons do not need a Prime Directive. Klingons fight for glory and honor. There is no glory in fighting a weaker civilization. It doesn’t matter what area that weakness is in. It would be the same as fighting a child. No Klingon would ever cause harm to a child, a child cannot defend itself. The very thought of anything like that would be an abomination. Klingons do not need a Prime Directive.
Klingons, not so Alien
Klingons are not as different as we think they are. The way Klingons order and view their reality is the same way humans did in their past. This is how Klingons fit into our human story. Thus, making Klingons an important part of humanity.