Well, if you have idle labour, digging a hole and filling it up again is a form of investment - it builds muscle mass and keeps your labour force fit and employed. It's a rather expensive sort of investment, but it certainly is an investment compared to the alternative of letting said labour force go idle.

Of course, if you can find more productive investments than digging holes and filling them up again, then you should be doing that in preference to digging holes and filling them up again. But the point is that when you have involuntary unemployment employing labour costs society nothing, so even paying the unemployed to remain exercised by digging holes and filling them up again is more productive than not doing it.

- Jake

If you only spend 20 minutes of the rest of your life on economics, go spend them here.