Let’s say I have an chat app. When an user logged in, I want to save their Id. Usually in a normal web app, I will save it inside javascript’s localStorage. But if I use this method in my chat app, when multiple users use my app, data inside localStorage will override each other. For example, if an user named with Id 01 logged in, and then got their Id is saved by localStorage.setItem(“userId”). And after that, another user with Id 11 logged in, and got their Id saved. Now, data associated with key “userId” will become 11.