In this case, it's about seeing Itsuki slowly lose her reasoning and becoming increasingly obsessed with Rin. Iruma is actually a very good writer and in the LNs the progression of her character from "I care for this student" to "I'm completely obsessed with her, without regrets, and want her all to myself" is extremely engaging from a writing standpoint.
It makes for very interesting character dynamics, where you have Rin who seems to be pulling Itsuki along into their relationship, but you realize that from the beginning she cared far too much for her than what was responsible for a teacher.
These kinds of "forbidden love" stories have always been a staple in romance writing, and part of why is because it requires a very different relationship dynamic than a normal one between people of equal status or power (two adults, two kids, etc.) which can be a lot more interesting in general.
You have to keep in mind that this is a fictional story, and it's not intended as a moral guideline. Nobody reads these kinds of stories and thinks, "Wow, this would be acceptable in real life," because it's obviously not.