The thermal breaker is there to prevent total loss of lighting in case of a short. The breaker will disconnect, stay off a few seconds, cool down, and flash on again, repeatedly, avoiding complete loss of the headlights until you find a place to pull over safely and diagnose the problem. A fuse or a fuse link would just cut the lights off completely at the first sign of an overload.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!