It's funny to think of developers and users as two distinct constituencies with distinct interests. I had thought any good developer, by definition, would do what's best for the user. Therefore the interests of both groups are the same.

But I suppose you're right that the average developer is their own damn person with their own interests which can only ever partly align with their customer's.

I suppose there are actually three parties, all distinctly interested: the programmer, the employer, and the end user. The programmer wants to get paid and do "good work". The employer is actually a multipart entity: management and investors, each of which have distinct interests. The end user is not a single such person but a mass of individuals, each with a different set of interests.

This is getting messy!

The individual end user can also be multiple distinct groups, some simultaneously:

Parents / caregivers / guardians and their charges, who can be further split in to children, teens etc etc.

Some benefits are for developers. And some are for the users. For example, many users, particularly ones who care about the look and feel of things, prefer when there is a smooth transition between screens in response to their actions.

You simply can’t do that with server rendering (barring hacks like Turbolinks, which are just approximating SPAs).

I agree, and I'm more critical of heavy front end Javascript frameworks that are part and parcel of SPAs. Things like the Turbolinks hack you mention, IMO, provide the best user experience, especially fast initial page load. But that can be messy to develop, so we end up with Angular or React which smooth the development issues, but make pages fat and slow for users.