Latex is the standard program for typesetting mathematical equations and is a very popular choice for typesetting multi-lingual works and music. Its plain text commands allow authors to write a variety of symbols quickly without clicking through many menus and allow authors control over formatting and alignment. Additionally, Latex is free, has good documentation, and has a large active community which have developed a range of freely available modifications. Thanks to the community, it is easy to find answers to common questions through Google and the Latex community actively answers new questions.
As a stand-alone program, Latex has a steep learning curve because you need to relearn how to do basic document formatting that is done using the graphical interface of WYSIWYG editors. Using Latex with Pressbooks makes the process easier, as you can use HTML for most of the document, only typesetting elements that are difficult to create without Latex. Latex can be incorporated into Pressbooks using the QuickLaTeX plugin, which allows shortcode to be used to include small snippets of Latex as well as provides a mode to interpret Latex syntax site wide.