Snappy is an attempt to solve one of the fundamental problems with Linux as a desktop operating system and that problem is package availability and package distribution. However, Snappy is not completely intended to replace debs. Snaps and Debs work alongside each other.
I am a Linux enthusiast and a project manager of a Linux application. While I love Linux systems as a whole, I despise the current state of package distribution. Snappy aims to solve this fundamental problem.
In Linux, packages are distro specific for the most part (it's possible to make one DEB that runs in all various different Debian based systems but that limits you in some ways) but not only that packages are distro version specific.
If I create a deb package for Ubuntu 16.04 then that package won't work on any version of Ubuntu. I also have to make a 14.04, 15.04, 15.10, and so on. These are JUST Ubuntu debs. I also need to make one for Debian. Then you need to make RPMs for Fedora 21, 22, 23, etc and those RPMs don't even cover openSUSE.
This means if I want to release a new version of an application and not wait on distro maintainers to include it in a repository (which usually takes an absurd amount of time) then I have to provide over 20 packages to cover the majority of Linux distros and still that's not going to be covering everything.
Ubuntu's Snaps provide a way to create one Snap that runs on every version of Ubuntu that supports Snaps. No longer distro version specific.
Snaps can be integrated into other distros. Potentially no longer distro specific.
Snaps are controlled in a repo that is intended to be maintained by the package developers so when we want to release a new version we don't have to wait on anyone.
Essentially, everything I hate about Linux package distribution will be solved by Snappy. Though it's important to note that these issues would also be solved by AppImages and Flatpaks.
Linux package distribution is awful for both developers and users. Snappy (also AppImages & Flatpaks) are intended to solve this fundamental problem with Linux based systems.
This question is really about why the move but if anyone is interested in learning more about what Snaps are and how they work. I created this video to explain the structure in-depth.