The Snap app in question is for the pulsemixer curses-based command-line sound mixer for the popular PulseAudio sound server, which is installed by default in Ubuntu 17.10 (Artful Aardvark) and other opt-in flavors, such as Ubuntu MATE. Pulsemixer will only be available as a Snap package in Ubuntu MATE 17.10, as the first step towards an all-Snap Ubuntu system. According to Martin Wimpress, the size of the Ubuntu MATE 17.10 ISO images hasn’t been affected significantly due to the installation of the pulsemixer Snap by the default, which was selected because of its smaller size and usefulness for Ubuntu MATE users, but also because it’s not available for installation from the official Ubuntu repositories, nor the Debian ones.