What does the "preset" layout config do?

[bsansone]

What does the "preset" layout config do? I don't really understand the description:

Description: Preset layout calculates intialize positions for nodes, and the force layout will start from the inited result. The quality of the force layout's result depends on the initial positions of nodes. Configuring a proper preset for a force layout will speed up the convergence of force layout, and enhance the quality in the same time. By default, the positions of nodes will be inited as grid

When would I use this?