# When would a GPU need to write data to a vertex (or other) buffer?

I'm trying to understand why, when and how a GPU would need to write data to some buffer inside its own Video Ram.

In Direct3D 11, there are two flags that concern this, which are D3D11_USAGE_DEFAULT and D3D11_USAGE_IMMUTABLE. The former gives the ability to read and write, the latter only the ability to read.

So, apart from the resources that imply actual rendering to, like back buffers or textures set to use in a render-to-texture scenario, is there any other case where one would prefer to use DEFAULT over IMMUTABLE? How would you get the GPU to change this data? (I guess from a shader?) Why is it not the default one the one that gives you read only access?