The angle of repose is the steepest slope at which a granular material like soil can remain stable without sliding. You can indeed get the coefficient of internal friction from its tangent, as tan(θ) equals that coefficient for a dry, cohesionless soil. But in practice, here in Haryana's semi-arid plains, our soils have a bit of cohesion from clay and silt—they hold a steeper slope than the pure friction angle suggests. Just last week, I had to explain to a client why his farm's excavated pit wall, standing at that textbook angle, still needed a gentler slope for the long rainy season.
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