They're typically just easy ways to submit early work, work in progress, etc to a conference with relatively little commitment.

I'm in the biomedical informatics world generally, and for my conferences it's a one-page abstract that describes your work, if accepted after peer review (which, there's a 90% acceptance rate) you then make a poster with more details about what you did and then stand by it for about an hour during the conference and answer any questions from people walking by. Then I typically just have links to the abstract and PDF of the poster on my website.

The poster doesn't prevent you from doing anything further... so, if you keep working on it, you can turn it into a talk or paper. And a lot of companies will pay for your travel to a conference if you're presenting, so it's often the lowest-effort way to get your company to pay for you to go to a conference.