More Microwave

Question

I see you are getting a little frustrated with the varuos questions about microwaves, but they are actually are very important. Can you please answer one more question on this topic. With regard to ovens you say that it is enough to run the self-cleaning cycle. Why then isn't it enough to thoroughly clean out the microwave of any stains and crumbs? Why do we need to boil the water inside it? What halakhic function does this boiling water serve? Thanks, Zvi