I can't speak directly from first hand knowlege of Frontier out of Denver, I can make a pretty good guess, based on common sence.
If an airline is to be competitive in areas other that just the price of a ticket, they must be competitive in their service as well. If Frontier, (or any carrier) has a presence in cities they serve, they must be able to offer flights from cities X, Y, or Z, in the early morning hours, when so many of the flights depart. To do that, the flight crew would end their day late, in city 'X', and depart in the early a.m. the next morning, with the pax that required early departure (most).
Comair is in the same situation you describe. They have one 'hub' in CVG. Until recently, all flights were back and forth from CVG, to X, Y, and Z. (More recently, Comair has initiated flights between city pairs that do not involve going in and out of CVG)
I live near MSP, and travel to CVG frequently as a non-rev to visit may son, who lives in CVG. I often will depart at 5:30 a.m. on a Comair flight. That airplane, has been on the ramp all night, having terminated its day of flying late the previous evening. The crew has a very early wake up call, as my son has piloted that flight on a CD line in the past, and he often told me that he would get to bed around midnight, and then have to rise at about 3:00 a,m,, to shower, eat, and get to the airport in time. Made for a sleepy morning.