Or, rather than worry about draining it to 0 before charging every day like you would a few years later once you wanted your phone to be on all day, you could only turn it on to return beeper messages, thus allowing many days of use before getting to 0 and recharging. That's the time period original commenter was talking about, and why it's different to the mid 90s / early 00s.

It wasn't many years that this made sense, but during the early 90s (and I think some of the 80s but not so sure) it was quite common to pair beeper for incoming with phone for outgoing. (At least in the UK, but I don't see why it would've been different somewhere like USA either.)