Sometimes someone's valuation of work is questionable. It may have had value to them, i.e. they paid someone to waste time out of pity, or their valuation could be wrong. They could pay someone to pray their stock prices increase not realizing it's useless.

There are much more obscure ways in which work can be entirely useless without anyone noticing, and plenty of ways people can end up with jobs that are pretty obviously a waste of time.