State management, URL fragment management, reimplementing basic controls...

One that I hate the most is that they first reimplement tabular display with a soup of divs, then because this is slow as a dog, they implement virtualized display, which means they now need to reimplement scrolling, and because this obviously breaks CTRL+F, they end up piling endless hacks to fix that - assuming they bother at all.

The result is a page that struggles to display 100 rows of data. Contrast that with regular HTML, where you can shove 10 000 rows into a table, fully styled, without noticeable performance drop. A "classical" webpage can show couple megabytes worth of data and still be faster and more responsive than typical SPA.

If you don't manage the history properly in your SPA, pressing the back button could take the user out of the app entirely.

If you don't let web developers manage history/state like this, we'd be going back to the inefficient world of, "every forward/back movement loads a whole page." (With lots of unnecessary round trip messages between the client and server while the user waits for everything to load).

Basically, the ability to manage history is a user-centric feature. It makes the experience better for them.