1) They can be less formal and lower overhead than a full multi-page booklet

2) They are a much more familiar and well understood form factor, as it is common in academic settings

3) The can be perused much faster, and they provide physical displays around which people can discuss/debate issues and ideas

4) They make for great social interaction and can be easily combined with a happy hour or party time to provide buzz and flow through a space

5) Posters are a lower overhead to create, as at the minimum they can simply be a blown up version of a one pager

If you really want something impressive looking -- get Samsung to donate fifty 70" flat screens to fill the space, and have a rotating set of posters throughout the conference. :)

-Andrew