Desktop mode and Longevity mode - thorough explanation

Hi there,

I've a question regarding Desktop and Longevity mode for batteries.

I quite understand the first one - I assume that it is better to have a battery continuously plugged in but charged at some constant level less than 100%, as it is pointed out in the description: "Battery will only charge between 50% and 100%". Or maybe this level will fluctuate between 50% and 100%? This is rather just a curio question.

But how Longevity mode works? From the description "Frequently connecting and disconnecting your computer to a power source without allowing it to fully discharge may reduce the longevity of your battery." I don't know how charging between 88% and 100% will help over here. Or to put it in another words - on my 2nd laptop which I move around with me, I charge the battery to 100% and it lasts for a couple of days. I usually charge it again when the battery level is 20-30%. Should I enable the Longevity mode? If so, then why?