For users: it makes software installation on Linux easier. If a project provides an autopackage, you know it can work on your distribution. You know it'll integrate nicely with your desktop and you know it'll be up to date, because it's provided by the software developers themselves. You don't have to choose which distro you run based on how many packages are available.
For developers: it's software that lets you create binary packages for Linux that will install on any distribution, can automatically resolve dependencies and can be installed using multiple front ends, for instance from the command line or from a graphical interface. It lets you get your software to your users quicker, easier and more reliably. It immediately increases your user base by allowing people with no native package to run your software within seconds.