Externalities, Pigouvian Taxes and Wikipedia It states on Wikipedia:


  A Pigovian tax (also called Pigouvian tax, after economist Arthur C.
  Pigou) is a tax imposed that is equal in value to the negative
  externality. The result is that the market outcome would be reduced to
  the efficient amount. A side effect is that revenue is raised for the
  government, reducing the amount of distortionary taxes that the
  government must impose elsewhere. Governments justify the use of
  Pigovian taxes saying that these taxes help the market reach an
  efficient outcome because this tax bridges the gap between marginal
  social costs and marginal private costs.


Q: When it says, "A side effect is that revenue is raised for the government, reducing the amount of distortionary taxes that the government must impose elsewhere", what does that even mean?
The Pigovian taxes are non-distortionary. For example imagine situation where government optimal spending is 100e and before Pigovian tax all 100e was raised through income tax which creates distortions on Labour market. Let’s say that after imposing Pigovian tax government gets additional 30e. Now since government needs only 100e for its optimal spending it can either choose to run 30e surplus or choose to reduce the distortionary income tax by 30e. The Wikipedia sentence refers to this second option where government chooses to reduce some other distortionary tax thanks to the revenue from Pigovian tax.