Some people are comfortable sticking their "good road bike" into a cardboard box and handing it over to airline baggage handlers.
Some people are not. Lots of people want to bring their favorite bike to ride the great mountain roads of Europe -- and feel they need to buy (or rent) a hardcase box so that they feel no fear that the fork or frame might get bent. Or unless you're more of bike mechanic than I want to be, even "minor" repairs to components or cables are a significant disruption -- not what you were hoping for your first day after getting off the airplane in a foreign country (? especially when your travel partners are eager to get going on other things than bicycling ?)
Not a problem for me, because my travel bike disassembles to fit into standard-size hardcase airline suitcases. No special charge on any airline. And my best road bike at home is a 20-year-old steel thing.
Also not a problem because I've enjoyed some of the great riding in Europe on rental bikes, and will gladly do so again if my travel road bike is not available.
Once by accident I did one of the hardest climbs in the French Alps on a rented city bike. Sounds strange, but in the Alps of France most of the famous mountain pass roads have been re-designed since the original Tour de France, so they're not so steep any more, just long slogs -- so I tried climbing one I'd never heard of, and got surprised. (If you need to be in a place where the famous climbs are steeper, try Austria -- ? or maybe the Pyrenees, I don't know them)
Mont Ventoux is outside the Alps by my definition: I am not volunteering to try to climb Mt V (from Bedoin) on a "city" bike.
Ken