You can potentially use it for a different route, but the price for the new flight will likely be eye-watering as it will reprice the new itinerary, including the original flight.

As an example, last year I was due to fly SJC-IAH-EWR on a ticket that was bought a few weeks in advance and cost about $300. A few days before the trip my plans change and I needed to instead be in IAH. I tried to change the flights, but the last-minute difference was high, so I simple flew SJC-IAH and skipped the 2nd flight. ie, a classic hidden-city flight, even though I hadn't (initially) done it deliberately. As a result, I ended up with a ~$150 FFC in my account.

However if I try and use this credit, the prices I'm quoted are insanely high, as it's repricing the whole trip and then charging me the fare difference. eg, I can book a SFO-LAX flight in 2 weeks time for $82. If I try and use the FFC from above for that exact same trip, the fare difference is $312 (plus the $150 FFC!) - despite both trips using the same fare class for the flight (GAA3AFDN).

Even if I try and use that credit to book IAH-EWR (the same route as it was originally for), the prices are high - because my original fare didn't allow a stopover so again it has to reprice the whole trip.