https://www.apple.com/batteries/why-lithium-ion/
You can read a bit about it here. The battery is designed to last at least XX full charge cycles (I believe it is 400 cycles for a iPhone). One cycle would be if the battery is 100% and you run it down to zero, then recharge to 100%.
So lets say you use your phone all day and the battery is at 50% and you charge it back to 100% overnight. If you did that two days in a row you will have used one full cycle. No problem.
Now you decide to run the battery down to zero for no reason (lithium ion batteries don't need to be discharged to maintain them) two times. You have just used up two battery cycles for no reason at all and shortened the life of your battery by two cycles. Not the end of the world if you do it once (still not needed though), but if you do this all the time like some people thing they need to, it adds up to a bunch of wasted battery cycles and a battery that does not last as long as it otherwise would have.