Why do professors give 'updates' about their unpublished, ongoing research?

The main reason is to attract interest to their current and future work, in terms of both public interest and potential students, scholars, and postdocs who might want to work with their labs. Talks are a good way of accomplishing this. Research isn't meant to be conceived, designed, done, and published in a cloistered off world. Anyone who believes this shouldn't be in research.

If the point of research is to advance/exchange knowledge and to improve life, then it deserves to be made a public, accessible resource. Perhaps even work that hasn't been completed. It's important for researchers to be transparent about their work, rather than trying to hide it, especially when they accumulate so much funding, some of which often comes from public, taxpayer sources (i.e. government funds).

You might think visitors are outsiders, but it's important to keep sight of the fact that they likely helped fund the research in some way, and more importantly, that they're part of the people your research should be trying to help -- and there shouldn't be such distinctions on this account between outsiders and insiders.

Science is a human activity. There is no point in doing research if nobody reads it. Your research becomes relevant once you can engage with people. Bill Thurston has an essay on this, focused on math, but I believe it can be extrapolated to many other areas: https://arxiv.org/pdf/math/9404236.pdf