It's not a project management app at all 
but a messaging app for team as chat and realtime communication replacement
We believe that 1) internal messaging–across PCs and phones, with file sharing, archiving and search–is a vital layer of IT in a “post-email” world, 2) the platform for internal messaging needs to be open, and 3) that we wanted Mattermost to be the leading project to serve as the open layer for internal communications.