Based on my personal experience, sometimes even if the API is yours, making changes can be difficult. The example I first think of is my last company.

We had a backend and frontend team, both working on the same product but the two teams disagreed a lot and would take weeks to make a small change even if it wasn't a breaking change.

To add to it all, there wasn't a standard to deprecating APIs so even a simple key rename could cause everything to blow up. Things weren't fun so I can see the need for these kinds of tools.

Sure, most people probably do. But some don't, and I guess in those cases it's good to have nice tooling to handle that.

Consider web applications without any backend using P2P technology, then you can't control the API, because you get sent data from other users directly. Once you deploy a new version, its useful to be able to transform that old data into the new format, if needed.

There is another pattern/benefit of such "client-side" transformations. You write your react components in a very generic way and rename / reshape data to fit them easily.