The point of this proposal is to bring this feature under the control of the browser, which is probably better placed to decide when preloading/prerendering should happen. It's already possible to do something very similar using JS, but it's difficult to control globally when that happens across different sites. Whereas when this feature is built into the browser, then the browser can automatically disable prerendering when the user (for example) has low battery, or is on a metered internet connection, or if the user simply doesn't want prerendering to happen at all.

So in theory, this should actually reduce bandwidth/battery wastage, by giving more control to the browser and user, rather than individual websites.