where -

I got one from Amazon.com.  The Personal Edition (PE) is for non-commercial use and is the same as the Standard Edition (SE) which includes a commercial-use license.

why -

It is a device operated by your non-mouse hand for panning, zooming, rotating, and moving up and down.  It is very sensitive and requires very little hand/finger movement. It takes a little getting used to but I think it is the PERFECT device for moving around in a 3D environment.

You still use the mouse for selecting and all menu operations.