On almost every phone I have owned, battery degradation is the primary complaint that I have after the first few years of use. Unfortunately, when the battery is just glued to the back of the screen, what should be a cheep and easy fix, has become impossibly risky. Instead, I am left nursing a dying battery to the point where, for one phone, I would carry an external battery pack everywhere since my phone battery could not last the day...

When I travel I bring a spare battery. Then I don't have to keep charging the phone all the time, and sometimes it might not be possible. With a spare that's one thing less to worry about.

The second thing is battery degradation. After a while the amount of charge will drop, and replacing it with a new and fresh battery feels like giving the phone new life.