UBI is not going to be enough to replace the income of most jobs. The idea, as I understand it, is that it's enough to cover basic necessities, so you don't have your population starving or living on the streets. People can take a year off to retrain without worrying about losing their housing. They can take up creative work that wouldn't otherwise provide a living wage, like writing novels, illustrating, various crafts, etc. They could, as Obs mentioned earlier, work half time instead of full time.
Also that (proponents hope) it slims down administration costs: it's a universal basic income rather than a means-tested benefit, so nobody needs to keep track of what the rules say you can have this month. Also, giving it to everyone unconditionally means none of it is ever withdrawn if you find work, so the incentive to work is never weakened by reaching a threshold that causes your income from benefits to be reduced: when deciding whether to work another hour, you'll always gain the full (pre-tax) market value of your labour for that hour.
As for the value of how people spend their time, it's related to the old problems of what GDP and so forth don't capture: that cooking our own meals and raising our own children is undervalued in what we can measure.