When I was young, there were books with characters printed vertically. One of my classmates told me that he enjoyed the vertically printed books because he could read a book with one hand, holding the book curled in his palm.
It was very convenient when he was pooping in a squat toilet while reading a book. It freed him of another hand.
In ancient times, China had books written on connected bamboo slips or wood slips.
A book was rolled up of bamboo slips. Even today, the Chinese call a book a roll of a book (一卷书）.
You can imagine the book was heavy in ancient times. When you read such a bamboo slips book, you put the roll on your left, extend the roll from the left, and roll it up again on the right, so you can read the book with your left hand unrolling the book and your right hand rolling the book up.
It is a very smart design by writing characters vertically on the bamboo slips. It enables you to read a heavy bamboo slip book.