Over time your heater cartridge's resistance changes slightly. PID tuning will help account for this and can also be used to dial in your hot end for printing materials of drastically different temperatures. To tune your hotend, connect to your printer and open a terminal. (tuning command below)

Tune it for the temperature that you most commonly print at - if you find yourself switching from PLA to Nylon, I recommend returning to the higher temperature. If you usually use part fans while printing, tune with the part fans running (M84 turns them on).

The command below will tune your hot end and save the values to the EEProm. Change the number after "S" to the temperature you want to tune for. Command: M303 P0 S210 X0