Pockets are one of the most popular features in tablet cases. They can carry pens (or stylus), paper, business and credit cards, headphones, sim cards, power banks, and even smartphones. The possibilities in number of pockets are many.
Messenger bags and backpacks all have pockets. Cases with handles often include pockets too as they are intended for travel. Portfolio cases will include multiple pockets of various sizes, while thinner folio cases include just a single pocket.
Another popular pocket is in wallet cases for phones. It's practical for storing money and credit cards while leaving your wallet at home.