[plt-scheme] inspectors; browsing and searching source code on PlaneT

I'm having a little trouble figuring out structure inspectors, sect. 4.5
in the MzScheme manual.  I've looked around, including a previous list
posting about 299.x, but it would really help to see an example of an
essential usage of a structure inspector.  Why do they take the particular
form that they do (which seems fairly arbitrary without knowledge of a
wider context)?
	Along those lines, I am wondering if there is any direct way of
searching and browsing source code on PlaneT, or at least downloading all
of it in one chunk to enable me to do that on my own computer.  I wanted
to find examples of usage as I was reading through the manual, and the PLT
sources (at least from what I can find through Spotlight on the Macintosh)  
do not have any usages of define-struct, among others.

Thanks,

Nick Chubrich.