It's a hobby pursued by bird watchers who want a better look, and a permanent record. Many avid birders own a spotting scope, and most everyone has a smartphone. The adapter puts the two together.
DSLRs are fine, and work in this application. However, the DSLR is much heavier, making a stronger tripod a necessity to reduce vibration. And the expensive DSLRs have a sensor that is bigger than the circle of light coming out of the scope's eyepiece. So your photos come out with a black circle around them. That said, lots of people use DSLRs for digi-scoping.
If you're working for National Geographic, you'd use different (and vastly more expensive) gear.