A Python "helper" script to do things XSL will not do, or cannot do easily.
Execute mbx -h to see the various command-line options.
Example: A Sage Notebook worksheet (*.sws) is a zip file of a few related files. XSL can extract the necessary information, the mbx script automates the process of creating the zip file.
Example: TikZ code for graphics images can be extracted and written into "standalone" files with XSL, this script will apply LaTeX to the files, creating a PDF, then optionally convert them via pdf2svg into SVG images.
Files for converting PreTeXt to Braille, see README there.