Ok, so, for a couple of you, "i" is there because itīs what assures conservation of probability. I find really delightful this answer. Sorry Bill K, I know that after all, the formalism has to be related to experiments and no matter how "delightful" it is, if it doesnīt match the experiments, the formalism is wrong. But, when I find that some part of a formalism is there only because it makes it match with experiments, I feel deep in my bones that there is something we dont know (and that feeling grows knowing that we are dealing with an incomplete theory -QM or QFT- that does not explain o lot of things like gravity, dark energy, etc.). However, when there is a way to justify from some previously "known" (whatever that means) commonsense, my bones dont feel that way.
In other words, I think that a great part of understanding is empathy. And The Duck response makes me feel happy and Bill K's makes me feel sad (Nevertheless, after all, it always makes me happy that all of you take part of your time to enlight me!).