Certainly there are complex UIs that require sophisticated front ends, but you probably aren’t building one, even though you’ve convinced yourself it’s necessary.
But if the requirements are for less page refreshes and high interactivity with always-fresh data, then client side rendering with a client side cache and cache management become important. Like Apollo Client, or what have you. Does it add unbelievable amounts of complexity? Definitely. Is it needless? Depends on the goal.
I would argue it's easier than manually polling for new data, managing the cache by hand and ensuring that everywhere the data is needed has been updated, and updating the DOM as a response to that new data.
So to answer your question: when done right, the user is served by having the most up to date information displayed for them at all times, with the number of network requests for said data kept to a minimum.
Is it worth it? I don't personally think so in the big picture, but in my day to day having this complexity live on the client side gives us so much more control over the UX that it's necessary to build what is expected.
Whether the user actually desires this experience is another question entirely...
The desktop app is dead. There is only the web app. Remember all those complex desktop apps that existed? They are all web apps now, and what would have been a new desktop app will become a new web app.
The only other app game in town is mobile, and people still use web app technology to build that even.
Enjoy your UIs, because they ain’t going anywhere, ever. There is no more ‘why’ in this question.