They boost the economy if there exist enough mechanisms that spread the generated gains to everyone. Those mechanisms used to be unions, political power of the working class and the demand for labor. The first two are close to nonexistant now, and the third one is being eroded at an increasing speed. The closer to human abilities automation gets, the less room there is for new human jobs to be created. At some point any new job invented can also be done by an AI, and at that point productivity gains have no mechanism with which to spread into the economy. At that point the owners of the machines will get everything they want from them and they'll have no need to pay other humans anything.