In Dragon Ball there exists the concept of a Kai. These include The Kaioshin, Grand Kai and the four Kais. They, as characters, serve to create (life forms) and oversee the universe, as opposed to seeking destruction like the Gods of Destruction do. Generally speaking, they hold the distinguished title of Kings of the Universe.
Within Universe 7 (one of the 12 universes in the series) there are five Kais. These include the North, East, South and West Kai -- all of whom have been entrusted with ruling over their own specific area of the universe and governing the actions of Guardians (such as Kami). These four Kais all fall under the leadership of a Grand Kai who oversees their actions. Essentially, you have four deities overseeing the four corners of the universe and even they find themselves answerable to a being that is higher in the pecking order.
You'd likely assume this being to be an awe-inspiring king who wears royal garments that are decorated in diamonds and embroidered with golden threads. Instead, you have a boom box holding, sunglasses-wearing hippie. This leads us to the first reason why there are so many of these beings in Dragon Ball.
Kais Play a Vital (and Fun) Part in The Storyline
Kais in the Dragon Ball universe are unique characters as they flip royal expectations. They fit perfectly into the genre, as shōnen anime loves to subvert viewer/reader expectations, and Akira Toriyama's Dragon Ball series is no exception. For example, you have a martial arts master who is a pervert (Master Roshi), a scary goblin-looking Namekian who is innately peaceful (Dende), and a loving king who chooses to dress like a teenager (the above-mentioned, Grand Kai). In summary, it makes sense that there are a lot of Kais, as they all add something different to the story.
The North Kai, also known as King Kai, is the king of the North Area of the universe. He assumes the role of Goku's mentor. This mentorship, coupled with bad jokes and cool telepathy, makes for an awesome character who has much to contribute to the storyline in terms of humor and wisdom. His immaturity also mirrors that of the other Kais, perfectly complimenting Dragon Ball's slapstick humor. West Kai's immature jibes at the expense of North Kai and his antagonism towards Goku also make for good humor and a healthy amount of protagonist-aimed hostility.
It's also worth mentioning the character Chronoa, who is the Supreme Kai of Time (a deity that can manipulate time). In Universe 10 there also exists a Kai called Zamasu, who is a Supreme Kai apprentice. He is an intelligent and skilled character; Goku notably says that he may one day reach the strength of Beerus.
Kais Exist in a Hierarchy
Another reason why there are so many Kais is that they exist in a hierarchy, with each character having a role to play within that compact hierarchal structure. Newbies who are unfamiliar with the concept of Kais will be surprised to learn that even Grand Kai operates under even greater beings called Supreme Kais, who govern the universe as a whole.
Making this hierarchy even more complex is the addition of Grand Supreme Kai (also known as Daikaioh), who is the highest-ranking Supreme Kai. If you're struggling to get your head around this order, it might be easier to think of it like this: Wherever there's a Kai there's a supreme counterpart. For example, East Kai and the East Supreme Kai. Accordingly, where there's a Grand Kai, there's a Grand Supreme Kai.
Kais Are Required to Watch Over the Universe
The main reason why Kais exist, as mentioned above, is because they have individual roles to play within the universe. The lower-ranking Kais watch over the living world but there are also Kais that live in the 'Other World' (also known as Cosmos) and oversee the afterlife. As they are characters that seek to create, not destroy, they're important for the continued functioning of the universe. As far as literary roles are concerned, Kais often assume the role of narrator to help explain the plot to viewers, so they are also arguably kings of maintaining viewership!
Finally, Kais serve as friend and foe to the protagonist, Goku. From King Kai as his mentor to Zamasu as his enemy, Kais are an integral part of Dragon Ball's overarching plot, not only for story development but for the development of the main character also.