so that you can create BOM (bill of materials), parts lists and inventories using the standard Visio report tools. Most of the shapes have custom properties (ShapeData) for manufacturer data, part numbers etc.
Below are some sample shapes for you to download and try out.
Visio Electronics - 721 Visio shapes and symbols in 39 Visio stencils (in both US units and metric units, all in Visio VSS, VSSX and VSDX formats) for creating your electronic circuit diagrams in Visio.