A lot of people on UC are single parent families, so for every batch of hours you work you migt have to cover some/all with childcare/babysitting.
I know someone that every time she worked a 4 hour morning, she would earn £3 takehome after the expenses of earning, she did it to get time away from children.
Also theres costs of meals, lunch while working for some people is more expensive than when at home.
and travel of coarse, i know people who for every day they work, 2-3 hours of pay goes towards fuel and parking.
And then theers tax of coarse.