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I feel like the gravel bike was invented because of helmets.
In 2000s-10s your standard bike for riding both road and offroad was the hybrid bike. Not everyone wore helmets at this time. It was seen as OK to go touring without a helmet.
Now in 2020s we are in the era of full blown helmet nazism. People will call you out on social media and youtube if you ride without a helmet. Now if you pick a basic cheap hybrid bike that people used to ride and combine that with a cheap helmet you will look like too much of a dork for cool instagram pictures.
In order to compensate for the dorkiness of helmets people choose to ride these cool looking gravel bikes that work both for off-road and road. With these type of drop handlebar bikes it not as dorky to wear a helmet because it's part of the "race outfit."