What are the most valuable things a PMM can give PM to improve product development, discovery and go to market but not limited to those? I feel the relationship is imbalanced since the PMM consumes more from PM than it gives back.

As a PMM, I was always conducting my own wide-ranging research about competitors and market dynamics (competitive and market intelligence) and providing this to PM. This included reading a lot of analyst reports, both about our category and adjacent categories, meeting with industry analysts to conduct inquiries on pressing questions of interest, keeping up on industry news, and understanding what trends are likely to impact the product and category. Finally, distilling this down to both proactive recommendations (where I would send unprompted suggestions to the product team) and reactive input (where I would comment on their product proposals and bring in this information) is how I would add value.

As a director of PM now, I have come to expect the same of my peers in PMM, as they well know. :-)

The PMM's job in this regard is generally to widen the aperture of PM. PM can often get trapped inside a very myopic worldview, where they are only building products for the customers or markets that exist today. In the worst case, PM can become a feature factory, literally taking customer orders for enhancements and just building those. PMM's job is to push PM to expand their horizon.

In sum: Adequate PMMs help market horizon one products. Great PMMs help product management build horizon two and horizon three products.