Because when you're working, you're creating 'value' for someone/something. That value is supporting your living costs - or at least going towards them.
The money the state is givng is paid to your childcarer. So the same amount money that taxpayers are contributing towards benefits now supports 2 people to live instead of one: because the 'value' of your work is benefitting - and so being paid for - by someone else.
More practically, imagine you become a childcare worker, and put your own 2 children into childcare to pay for it. Since you can look after 4 children, the same money from taxpayers means that 2 extra children get cared for during the day, allowing their parents to work themselves. The extra 2 children being looked after is the 'value' you have created.
People often get confused and think that spending money helps the economy. It doesn't. What helps the economy is for people to create 'value', ie work.