Nowadays SCRUM-Masters are in general much closer to the team than the team leads.
They observe soft skills like communication, collaboration, negotiation and behaviour on a daily basis.
Therefor they can answer the question how the team performs, where is potential for improvements of the team and individuals.

In fact due to their support by asking questions, caring for company's agile development, being a tackling partner they perfectly complement the team lead role.
They actively form the team and help it and all its members to grow, to go one step beyond.

That makes them more than a SCRUM-Master -
it makes them playing an undiscovered decisive role in the company's success.

It makes them a hidden servant leader.