Traditionally, professionals in the Apple ecosystem was a reference to video and photography pros (hence why those are the 2 out of 3 areas that they offer their own pro software, the other one being audio).
Sure, they can plug in other display, but the whole point of a laptop is to be able to use it to do good work on the go. Which is why a pro will prefer a laptop with a screen that can do what they need it to do.
Sure, they can plug in other display, but the whole point of a laptop is to be able to use it to do good work on the go. Which is why a pro will prefer a laptop with a screen that can do what they need it to do.