The well known part of Keynesian thinking is about the output gap. So if you give money to a formerly unemployed guy to dig a hole, you create the value of the hole in addition to all the other value the economy would create anyhow. This applies of course only if that guy is unemployed in the first place.

And as a rule of thump, every time I read a Austrian argument, I try to find the hidden assumption of markets working at full capacity. ( And every time I read a Keynesian argument I try to find the hidden assumption of an output gap.) Usually this is a nice way to understand the argument better.