I'm a high school math teacher. My students will text me their questions. I use the pen to handwrite responses to the students and then I send them the image. I also find that swiping on a keyboard is easier for me than using my finger.
Also, in the past I have used my phone to take pictures of student work. I have a chromecast in my classroom that I can mirror my phone to and display the work. I can then write directly on the screen with the s-pen to annotate the images. I don't do this much anymore as I now use an iPad Pro.