He uses the flamethrower because it self-reflexively allows him once again to be "the movie star" he always wished he could be, regaining usefulness once again via the proxy of his semi-failed movie career that is both literally and metaphorically embodied in the flamethrower. It is a totem for all Rick has lost, all he wished he could gain, and the tool that symbolically opens the doors of New Hollywood for him.
Of course, there's also the motif where the naturalism of Rick and Cliff's world is increasingly in dialogue with 'genre' overcoming their otherwise grounded reality. Rick and Cliff's arcs both mimic that of old Western gunslingers, while they, themselves, played old Western gunslingers. IE, Cliff's like an old retired gunslinger who gets brought to an evil gang's ranch, they hijack his stagecoach, he barely escapes in time, they invade his and his partner's homestead, and they defend themselves.
Likewise, Rick's an old gunslinger defending himself against the encroachment of civilization (ala Once Upon a Time in the West, this movie's namesake, complete with the mirroring crane shots) only to have one last hurrah.
If the flamethrower wasn't invoked, the whole thing this movie is setting up would have nowhere to go. It's the whole point.