I am dreaming and asking about that many years. This should be part of glare and bloom effect - dirt map on lens.
Here examples ArionFX with dirt map
https://arionfx.com/images/products/arionfx/xfp/maps.jpg
Dirt mask in UE5
https://dev.epicgames.com/documentation/en-us/unreal-engine/post-process-effects-in-unreal-engine
real life
https://i.sstatic.net/asfxX.jpg
https://snapgreatphotos.com/wp-content/uploads/2014/05/au0a7331.jpg
https://i.redd.it/fxckloslas541.jpg
https://s1.directupload.eu/images/260306/bp4aywk4.jpg
Submitted on AHA Ideas (CMAX-I-288).