Work gives one a sense of pride in accomplishment and soaks up our time. It is a social activity that matures us through forced interaction with many other different people. It gives a sense of belonging and inclusion. It stabilizes us.
If you're lucky enough to have all this from work, then good for you. Hopefully, you'll also have some energy left to do other fun things in your spare time, with all the extra money from work.
In practice, many people are working soul-crushing jobs only to (1) stay alive, and possibly (2) fund the thing that really gives them the aforementioned great things, for example art or team sports. In fact, a lot of people are already getting paid for things like basic scientific research, which does not generate any immediate economic value, but we value it as a society anyway. The same goes for art, for a few lucky ones at least. With UBI, I predict a flood of new science and art, as people of such tendencies no longer have to waste their talent on McJobs.