#1 is convenience
If I want to take a ride that starts in my driveway, I can ride a paved road bike. But, that means riding 55mph two lane roads without a paved shoulder and a lot of traffic. Yes people do it, but I do not enjoy it. All the roads off these high speed roads are dirt and barely suitable for a 28mm tire.
OTOH, if I ride a gravel road bike, I can ride the shoulder of the same high speed roads and be safely away from cars. I can also head down the dirt side roads and have miles of riding without seeing more than a few cars. It's still road riding, but has more hills (that's a plus!) And the scenery is roughly 1000x better. The bike DOES get dirty, but that's a small drawback for a much better riding experience where I live because there are far more places to ride a gravel road bike than a paved road bike.
TBH, if I lived in a different zip code where there was pavement on the low speed roads, I would spend more time on a paved road bike. Pavement is faster and I like going fast. I would miss the dirt road experience though.