Like Looslib says, the Windchill WT parts/gears really come into play when you start using Windchill for stuff other than just CAD data storage and control.
We do all our design release, design reviews, revision control, BOMs, procurement etc through Windchill. The gears effectively act as a place holder item onto which we associate/link CAD files, drawings plus non-CAD data like product specs/analysis reports etc.
When we come to release a CAD model/drawing, the designer promotes the WT part and all of the associated files, this process automatically sends emails to the Design Leader (and Project Manager if required by the project) asking them to approve or reject the release request. Once approved, Windchill then emails the project's Buyer telling them the part has been released/revised, it also assigns him a task to procure the number of parts required by the BOM and updates the part's status in the project's part tracking documents.
Building the BOM is fairly easy using the Product Structure Explorer function (its a little Java driven app), you simply drag and drop the WT parts in the structure you require for the BOM and type in the quantities. You can set it up to drive the BOM from the CAD structure but we rarely have our CAD assemblies sorted well enough for this.
The BOM structure also allows non-CAD users to explore a design by using Productview. The BOM structure is also transferred into SAP by some macros that we had developed but this is a little shaky and seems to need a bit of manual intervention to coax it along.
We aso have a load of standard report formats set up where Windchill outputs the relevant parameters such as no of parts in a BOM, the number released, dates etc which are fed into spreadsheets for the Design Team Leaders, Project Managers etc.
We don't use Windchill yet to track components once they have been manufactured or assembled but it does have this capability if required.
For the record, I'm not entirely a fan of all of this functionality, its great for big companies with big projects but Windchill does require a lot of admin so for smaller projects it can be more trouble than its worth. It all depends on what your company does but if you're in a dynamic organisation that needs to get things done quickly and doesn't plod through the same steps with every project then it can be very frustrating trying to progress things when you're badly constrained by things like not being able to get a component authorised for purchase because the Project Manager is off on holiday so can't approve the design release.
I'm told that Windchill 10 is much more intuitive but 8 & 9 are very unintuitive, especially for occasional users.
