It is already acknowledged that the initial guess can be wrong. We don't need to consider all possibilities to come up with a prior distribution because that is

It is already acknowledged that the initial guess can be wrong. We don't need to consider all possibilities to come up with a prior distribution because that is basically the 'trick' of the Bayesian updating scheme: Just start somewhere and we'll get closer to the true distribution by collecting more data and by using it the most logical way, namely by solving for the posterior. No imagination needed. It is more efficient to distribute the initial probability mass according to our best guess using a lot of imagination, but that itself is basically Bayesian updating because human reasoning is approximately Bayesian (or Bayesian with a noisy prior/bias). A physical interpretation might be that all the other realities are realized in terms of the Many-worlds interpretation or in terms of Tegmark's level 4 multiverse. Without much information we cannot really nail down which reality we find ourselves in (cf. the sleeping beauty problem and Boltzmann brains). But we can use Bayesian updating to become more certain about what reality is about. What do I gain from that? I am not entirely sure, but I find a probability is just better interpretable as frequency or fraction compared to a subjective quantity.