The change of numbers as the progress indicator loads is called a value change. The change indicates to users that something is happening behind the scenes — an important piece of feedback that lets them know their actions have had an effect.

Progress loaders aren't the only instance where value change is useful. For a real-life example, think of a slider for setting a price. As users move the thumb, the label moves with it, and the price value changes to reflect that. It helps users understand that the data is dynamic.

Loaders and transitions are examples of non-real-time interactions. Users observe the dynamic of value change, but it happens after their input.