1. Hover effects, you often want to respond to the user hoving their mouse somewhere, for instance showing a tooltip.
2. Battery/network saving, a site can pause animations or reduce update requests when the window is inactive.
3. I cant really think of a good use for this one these days, it was something browsers had in the 90s (not just readonly, websites could move your browser window where they wanted for a while). Maybe its kept for backwards compatibility.

Some ideas:

• for custom elements and general interaction, e.g., custom drag and drop or sliders
• for instance YouTube does this whether to auto play videos or to keep them muted
• i dont know, maybe for layouting things