That's not what I mean. You are asking for a reason I presume. In order to give an answer the question will need to be scoped/more focused. It feels like a questionnaire/multiple choice..
On sites like ..., for example you would get no response as it seems like "teach me Kalman fllter". There they use the Socratic approach.
And google is a good place to start.
IMO your best chance at an answer is if you are learning KF and you have a question like "I am trying to apply KF filter to GARCH/ARIMA with financial data feeds?" Something like that. In fact, you could give answers to your questions and post them. I reckon you will probably get more response.
Speaking for myself, my 2 cents.