A good poster session allows other participants at a forum to quickly find interesting research topics, instead of sitting through an entire lecture that may or may not be relevant to the participant's own work.
To catch someone's attention with a poster, it should be legible from at least a few feet away and have graphics that are easy to understand. Not all research details need to be included in the poster. Even if the researcher is not present, handouts can give more exact explanations for those who are interested.
In additions to presenting research, poster sessions are often used for networking purposes. Young scientists nearing completion of their doctoral theses often present research at poster sessions in the hopes of meeting potential employers.