Lets you use your phone for longer between charges. You're acting like battery life isn't a user feature, but it is: companies advertise heavily on it, and it can often be a differentiator between midrange and premium phones.
You're looking for a user story where only looking at the corollary makes it obvious that battery life enables everything else.
Suppose you shipped a phone with 99% less battery life than its competitors. You could, at trivial expense, increase your 1% to 10%, your engineers propose how to do so.
You reply, "My phone lasts longer and that lets me _____".
Nobody is wandering around wanting their phone to last longer purely for that sake.