Again, …, SketchUp Free is not an entirely new app. It has the same core as the SketchUp desktop version. The little html GUI around is rather little effort in comparison of the work and technology that has been invested into the core for probably over 15 years.
You asked why does SketchUp do this? They did probably only not publish “SketchUp Free” to annoy you, but also because they had the technology already available (remember “my.sketchup”, then “SketchUp for Schools” on Chromebooks?) in order to stay relevant for young generations who grow up in a multi-platform world.
If you knew something about opensource and agile software development, you would have heard that there are ways to release products (fixed release or rolling release) and that they significantly impact success of projects, the speed of development and quality. Some of the world’s best software is developed as rolling release (with fixed long-term support releases, google it). Rolling releases are brought earlier to users and better tested. Fixed release schedules however are prone to delays and are tested only very late, which results in too long feedback cycles to the developers.
My speculation is that the SketchUp core is now developed completely independently from Pro features (likely separate teams), and continuously released in the browser for testing and tuning performance. Pro feature development can be more focused. The biggest benefit is for paying customers, even two-fold, a well-tested engine and finally more features that get ready before a yearly release.
Users who prefer the free version pay by opting to test-drive the core and new UI experiments (google A-B testing).