Imagine this scenario:
I've developed my app. Designer gets hired later and spits out a cool new layout and design... No feature changes at all.
similarly...
Then next year, management decides we need to give the app a "fresh new look" for v2.
[If I'm using React or JQuery-only] From what I can tell, this will cause me to have to dive through a whole bunch of JS code where I'm sifting through business logic and imperative code to find JSX/html sprinkled throughout and make the necessary changes. Also, since I'm changing actual code the chances of introducing new bugs in this process is high.
[If I'm using Angular] Go through html. Change html and css. Leave code and business logic alone.
This is not an entirely contrived situation (at least not for me). As I'm working towards a releasable product, one of the areas that go through the most change and flux is the design and layout.