Are filter drivers intended to extend system drivers?

Question

Are filter drivers intended to extend system drivers?

Is this their main purpose?

Are they basically just an extra layer that sits between the driver and

Answer 1

In a way yes.

For example, if file system filter driver is for file encryption/compression/security, it is enhancing the file system functionality.

The filter driver does not handle talking to actual devices. They rely on lower level drivers to communicate with device. The filter drivers are add-on to the drivers to implement certain functionality. The active drivers which modify data/request are to enhance vanilla drivers while the passive filter drivers are just pass-throughs without any direct enhancements.

So I think your assumptions are correct. Will like to hear different views though.