The batteries used in phones degrade quickest below 20% and above 80% of charge. This is already at room temperature. When the batteries or phone is hot the degradation at those state of charge (SoC) is even worse. In the car when using CarPlay the phone can get really warm, especially if it is in the sunlight which tends to happen easily. When left in the shade it gets hot due to restricted airflow.

Therefore I would like to limit the SoC on my phone to not excessively degrade the battery. I very rarely, if ever, need more than 80% of the design capacity of my iPhone anyway. So I prefer it to last longer.