Chargie promises to prolong the quality of your phone’s battery life.
To be honest I don’t know why. Manufacturers from Samsung to Apple to Microsoft say is the case, but I haven’t managed to find an answer with a better source than itself.
I get the impression that it’s about staying at high levels of charge.
For example, this page: https://support.microsoft.com/en-us/surface/caring-for-your-surface-battery-9ccdfa7b-d074-f629-425c-1c090ac66bed
So when I go to bed I put my phone on the charger. After about 1.5 hours it is at 100%, then it spends the rest of the night on the charger at 100%. It spends 1/4of its life on the charger at 100%. From what I read of this, if it was taken off the charger and allowed to discharge, it would be less of an issue than having it at 100% then every time it drops to 99% the charger tops it up to 100% again.
But TBH I have not found anything that backs up the idea of charging to 80% only vs charge to 100% and taking it off the charger (you can get apps that give you a notification when it gets to a certain charge level). Lot’s of people and articles talking about it as a known fact, but not so much in the way of sources.