Many users do not have stable internet connections (ships, rural workshops, underground data centers). SketchUp 2017 works entirely offline after installation. The web version does not.
While SketchUp 2017 is a hero for free software, you should know its hard ceilings. sketchup 2017 free version
The modern web-based SketchUp Free does not support third-party plugins. This is a deal-breaker for serious makers. Many users do not have stable internet connections