Bug and features updates are the big updates that take up to an hour or so to do. These are the updates that add features like more emojis, better connectivity, or changes the appearance of the menus or the operating system as a whole. (ex. IOS 8 to IOS 9 or Android Lollipop 5.0 to Marshmallow 6.0). The manufacturers tend to put a lot of security fixes in these too but just hide them since the issues were not discovered by hackers and they want to focus on the feature updates.
In short... all updates to mobile devices are important!
iPhone & iPad