Shader Motion Vectors - what do they do?

[BOXOPHOBIC]

URP has a time based Motion Vectors pass in the shaders (created with SG or ASE). When debugging with the rendering debugger, only animated transforms show up. When is this pass used? Tested with Motion Blur post processing enabled.

[BOXOPHOBIC]

To give more context, the cube is rotated via transform anim, while the barrel is having a sin based vertex animation. Only the cube wtites motion vectors:

Edit: After saving the SG again, the MV started magically appearing…