What is the purpose of a 'seed source'?

I have completed the tutorial "Incompressible flow through a pipe" but I'm uncertain about the function of the translated face (the 'seed source')? What's its function?

Particles source

The 'seed source' is where the particles (that form streamlines) are released. A seed can be any geometry entity whether it is part of the flow volume or, as in the tutorial, not. For more details see "Particles or Streamlines".