Because it’s a fancier train (which generally means bigger windows for a more panoramic view on a scenic route) and it provides a running commentary about what you’re seeing on the PA system. Other trains follow the same route or, in the case of the “Heidi express,” nearly the same route. You should be able to get off the Bernina Express and continue your journey, using your railpass, on another train if you want to.

I’d suggest spending some time with a good guidebook for Switzerland and then some time on one of the railpass travel agency sites looking at options for Switzerland. Some of the scenic railroads are privately owned and not covered by a eurail pass while they are covered by a Swiss pass. There are numerous options for railpasses, discount cards, etc. in Switzerland that may be a better deal for what you want.

We’ve been over that route a few times (I like the scenery) and I would not pay more for the fancier train. The panoramic windows just stretch up into the roof a bit, but the scenery is out the side windows. They only run a very few fancy trains a day, so if you want more flexibility in when you start, take the regular train. I seem to recall that the luxury cars had fixed windows, while the regular cars had windows you could open to take pictures.

We stayed only in Chur, which was nice, but it is a city, not a village. We took a train from Chur to Arosa (spelling not guaranteed) and back, a nearby ski resort, and it too was a very scenic ride. The ticket we had also allowed us to ride a couple of cable cars (it was not ski season).

I don’t know where you are going after Chur, but if it is Luzern, there is a way to turn a mundane ride into a scenic ride by specifying a route through Andermatt.