https://docs.unity3d.com/ScriptReference/RenderTexture.DiscardContents.html

hint the GPU driver that the contents of the RenderTexture will not be used.
on some platforms, it can be good for performance if u indicate when the current contents of a RenderTexture are not needed any more.
this can save copying it from one kind of memory to another when the texture is reused. Xbox 360, XBox One and many mobile GPUs benefit from this.

This call is typically only meaningful when the given RenderTexture is currently an active render target. After this call, the contents of the RenderTexture are undefined, so the user should not attempt to access its contents before either clearing the RenderTexture or drawing into each pixel of it.

Both the colour buffer and depth buffer are discarded by default but either can be selected individually using the optional boolean parameters.