If you're organizing a conference, you'll need more than a simple assembly hall or space with a lot of chairs. The hall should be located in a building with an attractive façade and plenty of parking - participants in your conference will only be with you for a day or two, and making a good first impression is vital. You'll most likely need equipment for projecting video and sound (and you may need simultaneous interpreting capabilities). You may also need a separate space for registering participants and coffee breaks.
The best exhibition spaces have plenty of room to help people circulate. For example, a café filled with small tables wouldn't be considered an exhibition space in the traditional understanding of the term!