You might read up about the history of the communist block, where such a system was in place. The government was literally obliged to give every single person a job, and it caused a ton of problems.

Seriously, UBI is better than inventing jobs just so that people could have jobs. At least in this system, you can get cash, and become a home maker, an artist, or invent some other occupation for yourself that you wouldn't otherwise have. Unlike the "guaranteed job" program, where you would be pushed to do barely useful stuff, that not only would break your spirit, but would not give you time to do something truly productive.