While Unity Terrain is limited to 32 control textures, Texture Clustering turns each control texture into 3 textures which are height blended together and varied by noise. This creates a complex and procedural surface which doesn't show tiling artifacts, and can have more variation than a standard texture. You can also use 3 smaller textures instead of one larger one, increasing quality and saving memory at the same time.
The new Stochastic mode allows for a texture cluster look without needing additional textures. While using multiple textures can produce a superior look, this mode doesn't require the extra memory or authoring of textures- simply turn it on and it will generate variations on your texture in the shader
When multiple textures are clustered, easily control the noise function with a small texture, and control the blending between the clustered textures separately from the main blending. Per Texture controls are available for each control texture, allowing you to fully parameterize each cluster to get the maximum quality.
Don't be limited to repeating textures, use texture clusters to make complex surfaces which never repeat, or create greater variety across your terrain by blowing past the 32 texture limit in Unity