Reading an article about multistage rockets (not educated at all on the topic), and from the get go it seems to assume they are used/need to be used.
Another reason is that each motor stage can be designed for different purposes to match the flights characteristics.
So, a relatively short high thrust burn to escape gravity
And a longer or continuous burn to accelerate over a course / trajectory
Can save weight and complexity so adjustable nozzles are not always needed.