Re: Why upgrade?
I'm struggling to remember anything significant I've got from an OS upgrade on my previous android phones ... Security upgrades are obviously important, but for other things?
Two things that spring to mind are
-- Lollipop relaxed the access control mechanisms employed on external SD cards that had prevented third-party file manager apps from writing to the card in KitKat.
-- Marshmallow allows the user to edit the permissions that an installed app is allowed to use, previously the user had no control.
Those are both worth upgrading for.