michael-jeulinl/MSVTK — comment on commit d742be7395cc01dec9c016b5eae1a80597f5415b
File:  line 0
By finetjul:

This is VTK's philosophy of doing things, that workflow applies with extents. Feel free to refer to the book to read more about filters and pipeline.
The user controls the final view (the mapper, the end of the pipeline).
Said otherwise, the mapper (directly visible to the user) is the public API, upstream the pipeline is just implementation details.
What if your mapper, is a mixed of 5 of your readers, you would have to manually control each of them. If you only control the mapper, it propagates to 5 of your readers.