Agreed. But it isn’t quite useless. In “On Writing” Stephen King states that he got the original idea for the story while at a school, and saw that there were period pad dispensers in the grammar school bathrooms. He made a comment along the lines of “oh is that really necessary at that age”? And in response someone else replied and used the phrase “plug it up”. That is how he got the original idea, and what the girls chant in the locker room; plug it up. Having Carries mother called to the office because of this, is also essential to the storyline.

Carrie White getting her period helped her powers manifest as she crossed the biological threshold between prepubescent and pubescent, and was also completely essential for the climax. The reason that pigs blood was dumped on her to begin with was because of the period scene, and without it, the novel would have been completely different.