Vertex pipelines transform raw 3D geometry into the 2D plane of your monitor. Vertex pipelines also eliminate unneeded geometry by detecting parts of the scene that are hidden by other parts and simply discarding those parts.
Does It Matter
Not really. I honestly can't think of a single game in which vertex processing capability is a limiting factor.
Miscellaneous
The GeForce FX series uses one large large vertex processing array. I'm not sure how many 'standard' vertex pipelines this corresponds to.
Also see Vertex Operations.