What is the reason the Night King from Game of Thrones would create his generals (?)
The Night King's "generals" perform specific tasks for him, as well as aid and potentially protect him during battle.
To explain how there's purpose/value to the Night King's generals, it's first necessary to separate the generals from the rest of the Night King's army in two major ways.
1. The generals appear intelligent, whereas the wights are basically mindless zombies that run rampant trying to kill anything and everything that's alive.
2. The generals have unique weapons and are quite skilled in using them. The wights on the other hand all have more rudimentary weapons and are, for the most part, easily defeated in [one-on-one] combat.
That being said, the generals are able to support the Night King in ways that the rest of the wights simply aren't able to.
S07E06 ("Beyond the Wall"), just after the Night King spears down Viserion, one of the generals immediately anticipates the Night King targeting the second dragon so they fetch him another ice spear without needing command.
S06E08 ("Hardhome"), when one of the generals specifically seeks out Jon Snow. Not only did this appear to be an order from the Night King (since he was shown watching from above after the fight was over) but also the general was extremely skilled in combat having easily taken out a Thenn and [arguably] almost defeating Jon Snow (if the general wouldn't have been surprised that Jon Snow stopped his sword).
Lastly, aside from when he's riding a dragon, basically every time the Night King is shown he's surrounded by 3-4 of the generals. So, given that they're intelligent acting and fairly skilled in combat, they at least serve the immediate purpose of being bodyguards for their King during battle.
If they are killed, the wights they created are also killed. So, more generals means higher chance of a general being killed, meaning higher risk of losing wights.
Since the generals mostly serve to protect the Night King through proximity, it's very likely the case that they don't kill many people, and thus don't have a large number of wights "tied" to them. If their main purpose is protecting their King, they simply wouldn't have the opportunity to kill so many living humans.