On one end of the spectrum, they make capital available to businesses in order for them to bootstrap and expand. They pick winners and losers and make decisions that significantly alter the course of economic development.

On the other end of the spectrum, they gamble on tulips. https://en.wikipedia.org/wiki/Tulip_mania

There's everything in-between. Factor in the universal human tendency for self-delusion, and you got tulip pushers who think they are moving the pillars of world.

The counter argument (Mariana Mazzucato in particular) is that they extract value rather than create value, and this, like all other value extraction, is a net harm.