In Books and the Teenage Reader*, G. Robert Carlsen devoted a chapter to what he called "subliterature". In it, he suggested that reading formulaic series books like Nancy Drew as well as comic books was an important step in the reading process for most children.
The theory was that when a reader sort of knows what will happen next, she/he can relax and get lost in the story rather than focus on the reading process itself. Reading becomes subconscious, transparent.
In Common Sense Media's How Comics Helped My Kid Love Reading 7/14/2016, Sierra Fliucci writes about how graphic novels have filled a similar role for her children. I am sure many parents have had similar experiences with their own children. Whether the books are the Lunch Lady series or Big Nate, magic happens when the reading act becomes transparent and the story foremost.
This is why we should never apologize for making "subliterature" available to our libraries, e-book collections, or classrooms. Kids who read a lot, read better.
There is no real secret to improving reading abilities as much as the producers of high cost reading "programs" would like educators to believe.
* I was fortunate enough to have Carlsen as my adolescent lit professor at the U of Iowa back in 1978. While now dated, the ideas he presented in this now out-of-print classic have stayed with me for many years.