If there is some requirement to contribute to society in some way (work, volunteer, go to school, create art, start a business, teach, mentor people, join the military, etc.), the government could use the program to encourage people to contribute to society in specific ways.
If work is required for a UBI, it goes against the premise of a UBI which is provide a living wage for all without work needed. It therefore becomes a “fast track” of sorts to get a job. However, we currently have more positions available for work than unemployed. If the person wanted a job, the opportunities are available right now and a UBI wouldn’t be needed.