I don't understand, Apple is now charging us developers to just use Xcode?
No. XCode Cloud is to run tests of your code in the cloud and report the results to you.
Look at how it is described: "continuous integration and delivery service"
This is valuable if you have a large set of tests; after every change the tests can be run so that you can learn rapidly what broke as the result of a change. And tests don't have to be pass/fail; they can also be tracking things like performance or energy or network usage, so that again you can catch regressions in those areas.
For small apps this may not be necessary; run the tests can be run on the developer's primary machine or a secondary machine. But as apps get larger, the time taken to run the tests can grow to start becoming a real impediment.
In principle, a second use case would be if your tests were able to run against all manner of different hardware and OS versions, so basically press one button and tests run for every piece of HW and Apple OS of the past five years. I don't know (I don't think so) that XCode Cloud has that sort of functionality yet, but I expect that's one direction they will head.
Of course all this requires that you write your code with many test cases...
But hey, even Apple can't make you behave like a professional if you refuse to behave like a professional...
There's some additional stuff involving testers, for example how new builds (or automatically failed test builds) get distributed to various people, which is convenient but less dependent on having large remote compute power.
This is best thought as a very first preliminary step in Apple selling generic cloud compute services to users; it's not an end point. More developer functionality will surely be added, along with more ways to buy additional cloud compute; but everything takes time...
