Advertising benefits
One likely benfit of this new fare type is that is makes it easier to advertise cheap fares, as you can often pick up extra revenue through checked baggage fees. In today's environment of frequent price based promotions this is useful.
Fo example with a $10 baggage fee (plus a $10 surchage if you check in at the counter) its a bit easier to advertise a $1 fare, knowing you will either save a bit on baggage handling or make it up on baggage fees.
Equally the $59 fare becomes advertised as a $49 fare.