The main reason is who want to match \usepackageminion with matching math. The minion package works with the (free) Minion Pro fonts often bundled with Adobe software—but those do not include the math glyphs. Minion Math fills that gap, but at a cost (~€100–€300 depending on license).
: These are free, high-quality OpenType math fonts that work well with classical serif text. Minion Math Font Free Download
is a commercial font family and is not available for free download legally. It was developed by Johannes Küster (typoma GmbH) as a professional mathematical add-on to the Adobe Minion Pro typeface. Purchasing and Licensing The main reason is who want to match
Integrating it into LaTeX workflows can be more difficult than using standard free packages like mathpazo or stix . Minion Math Font Free Download