Contracts are the legal obligations the convention organization enters into in order to operate. The most important ones are typically for a Convention Center or hotel function space, one or more hotels for sleeping rooms, and Hospitality catering contracts. There may be many other contracts, depending on what type of functions a convention intends to conduct.
Any time the convention intends to bind its actions and the actions of other people together in exchange for money or other consideration, a contract of some kind should be contemplated. Negotiating the terms of the contract can be complex, and should be handled carefully. It is critical that the convention understand what it needs, and what is and is not a "good deal" for them.