The idea of not charging all the way is to get more total use out of the battery.
These batteries degrade with use. They degrade faster when they have a high charge or a low charge. Suppose each day you used 50% and charged at night. That's a cycle every two days.
If you are doing that by going from 100% to 50% and then charging back to 100% you will get some number of cycles before you need to replace the battery. Let's say it is 1000. You'd need to replace the battery after 2000 days.
If instead you do your 50% per day by going from 80% to 30% and then charging back to 80% you are operating in a charge range that has a lower degradation rate, so now you might get 1500 cycles, or 3000 days.
I understand it increases the total lifetime of the battery, and certainly I do know a few people who are still using their iPhone 8, but most people seem to replace devices every 4 years at least. I just don't find the tradeoff worth it, but I understand from some of the sibling comments that this is probably most useful for people who don't typically run on battery power.
These batteries degrade with use. They degrade faster when they have a high charge or a low charge. Suppose each day you used 50% and charged at night. That's a cycle every two days.
If you are doing that by going from 100% to 50% and then charging back to 100% you will get some number of cycles before you need to replace the battery. Let's say it is 1000. You'd need to replace the battery after 2000 days.
If instead you do your 50% per day by going from 80% to 30% and then charging back to 80% you are operating in a charge range that has a lower degradation rate, so now you might get 1500 cycles, or 3000 days.