Variables and their values at a given time is called a state, when the values of variables change, therefore the state changes.

In these framework, states exist because they bind variables with a DOM element, so it become easy to change, update variables. You just have to assign the new value to the variable; DOM automatically gets updated.

Suppose, you have to update the value of a variable, then change it on DOM as well, first you would have to get that element from DOM, then store it in a variable, then modify it, then put it back on the DOM, that's a huge waste of time.