This forces the PS4 to reconnect to PSN to establish the correct time-a routine check that happens every time you try to play a digital or physical game. If that battery is removed for replacement or just dies, the system can't properly track the real-world calendar.

The problem is the PS4's CMOS battery, which fits into the hardware's motherboard and is used to internally track the date and time, even when there's no power.

Further Reading The looming software kill-switch lurking in aging PlayStation hardware Connection problems