In general, widebodies cost more money per seat, and are not more fuel efficient than maximum narrowbodies.
The only cost economy that widebodies have is 1 cockpit crew vs. 2 crews for a pair of narrowbodies. And airport slots, if applicable.
In the USA for example, domestic widebodies are not cost competitive with narrowbodies. They are just repositioning flights or utilization flying.
Or if the airport is slot-restricted, which applies to JFK in New York---one of only 3 airports in the USA (LGA and DCA are the other two) with FAA-imposed slot restrictions. Since you can't increase frequencies, you need to increase the size of the planes. This is why from JFK to LAX and JFK to SFO, Delta flies wide-bodies or ex-TWA 757s.
As for domestic, it needs to be defined. Air France, as someone has said before, needs wide-bodies for routes to its overseas departments (Air France flies 468-seat 77Ws to its Caribbean and Indian Ocean overseas departments or 275-seat A340s to smaller overseas departments, depending on whether the route is from CDG or ORY, and British Airways needs them to the Caribbean overseas territories and Bermuda (although an A321neoLR could work to Bermuda). Russia needs wide-bodies for routes between Moscow and Vladivostok; Transaero before it shut down employed non-ER 773s between those two cities. Likewise, in the USA, JFK or EWR to HNL must use a wide-body (UA uses an 764 while HA uses a 332).
BTW, someone mentioned LHR-EDI---might BA's 767 utilization depend on the cargo needs? It seems as though the one-class 767s always fly with a full belly cargo load.