its bcz they wanted naruto to grow up and to create his own ninja way
bcz at the start of the arc naruto was scared s**t less when the demon brothers attacked and sasuke was the one that saved him and sakura. thats when naruto made a promise that he will never back down and run away
in the final haku/ zabubza vs team 7 fight. haku explained that the reason that he is fightin is bcz he wanted to make zabuza's dream a reality that was his reason to live. after haku was killed by kakashi, naruto did his speech to zabuza and zabuza commented back wit this
"While he fought you, his heart was breaking in two… u see haku, was always too soft and too kind, he felt pain and sorrow now curse him I feel them too…. Something else I feel content this will end……… well cat got your tongue….. are u so surprise to discover that I am human… even shinobi are human no matter how hard we try to esacpe that simple fact, we always fail….well at least….. I have failed"
[ *sniffles* TT^TT is beautifull ]
so the only reason y they had the zabuza/ haku arc is to build the foundation of naruto's ninja way.