The backend runs the tv tuner cards, handles scheduling of recordings and runs the database. Like I wrote way back in the original reply. Mythtv doesn't run without at least one backend connected. Mythtv is a PVR suite of applications : if you don't need a backend at all, because you just want to play local files, then you don't need mythtv. There are lots of simple file-player applications out there --any of them fit this need better than mythtv does.
+1. I'd recommend XBMC if you don't need PVR functionality