There’s not a singular most important function of a Scrum Master. The most important or most valuable contribution is dependent on the context.

In my experiences, the most common requests are around helping to remove impediments, facilitate the Scrum events, and making sure that all of the stakeholders (including the Product Owner and Development Team) understand how best to apply their skills and knowledge in an agile environment effectively. Although they don’t have to be important since they are common, these are the things that teams tend to struggle with, and help is often a more immediate value addition.