In which case… why not just give people the money?
First is psychological. People respond well to being productive, tend to not respond well when they’re not. One can derive from studies and take the position that increasing the welfare state would increase substance abuse problems, for example. There’s no path forward for these people.
Second, if you’re working a JG, the goal for many would be to get OFF a JG job and into the admittedly shrinking set of private sector jobs, because they will undoubtedly pay better. So, the JG might be giving the individual useful job skills and experience that they can then use to move into the private sector. Obviously, the best source of candidates for a job opening would be somebody who has already been doing the same job on a JG.
People like goals. They respond well to them. Not so well to a life they see as hopeless. So, if the JG gives the person the ability to reach for the brass ring….that’s all good for society.