If you are a primary teacher and you don't use book bags in your classroom, I highly recommend them. I have always used them. When I taught Kindergarten my students would take home the little phonics and sight word readers that we used in class. Rereading the books with their family was part of their nightly homework. As the book bags started to get full I would have students take the books out and keep them at home rather than returning them. Another fun thing the Kindergartners always liked to do was buddy reading. Since they had reread the books so many times they always felt confident and successful. Those who did struggle could always be helped out by a friend since the books were familiar to them as well.
I used the book bags differently with my first graders. They would take a book home every night, which was at their Accelerated Reading (AR) level. When they finished the book and were ready they would test on the book. This was a great way for students to work on comprehension and to make sure they were progressing in their reading.
At one point I taught an after-school program. These students were not really given an actual "book bag," but they were given a reader to take home each night. The back page was a signature page. They had to read the book to 10 people and have them sign the page to prove the book had been read to them.
Another way I have used book bags is with struggling readers in my classroom. In this case, I would choose a book that I wanted them to work on for a period of time. It varied between Dr. Seuss books, phonics readers (I Love Reading), Houghton Mifflin Phonics Library books, or just a beginning-level book. My students would take these books home every night and were supposed to read these books with their parents. Unfortunately, for many of these students, it often did not seem like this was happening. Having the book bags was still helpful though. They knew what book they were supposed to be working on, so when they had the time in the classroom they could read independently, with a partner, or with me.
Ok, this is way more than I planned on writing, but all this is to say that book bags are a very useful tool in helping students build their reading skills. It sets an expectation for them as readers. It is enjoyable, especially if you let them pick their own books and the books are at their level. It helps strengthen comprehension. But, best of all it is easy to do. I hope that it is something you will consider doing if you aren't already.
If you are interested I do have a book bag packet available at my TpT Store. It is a simple little resource that provides a parent letter and a retell sheet for students to use. The parent letter is in both English and Spanish. I've provided the link for you in case you would like to check it out.