The poster is an advertisement for the paper. Same goes for oral sessions. Almost all talks include the phrase "for details, see our paper". Posters are a nice

The poster is an advertisement for the paper. Same goes for oral sessions. Almost all talks include the phrase "for details, see our paper". Posters are a nice middle ground between an abstract and a full paper. Spending 5 minutes at a poster talking with the author is almost always enough information to decide if you want to read the paper carefully. This is especially important in theoretical fields where it can take hours/days to fully comprehend difficult proofs. Poster sessions are also important for meeting people in your niche.