Mobile middleware, in the IT sense, consists of server or client software that is designed to allow users access to extensive applications and data -- usually residing in a central information resource. This middleware software allows mobile users to reach in and retrieve specific snippets of information or update targeted sections of a database. Typically, middleware can involve groupware, messaging, customer relationship management (CRM), sales force automation (SFA), enterprise resource planning (ERP) and other applications. Middleware servers can also be used to restrict access and provide security levels to protect that valuable information resource.
Do you need middleware servers to access information from a central site? In most cases yes, and besides you really wouldn't want to provide direct access to hundreds or thousands of mobile workers since each one would have the ability to directly impact the validity of that data, and there would not be much control on how much and what kind of data they can access. You can, however, avoid middleware when using a locally hosted application, such as the non-enterprise version of the Blackberry application. Since you are working within your own information world, you can call the shots and restrict access to whatever data is on your product shelves.
For more information on this, research the Web sites of vendors who specialize in mobile middleware. They usually will explain why it is important and what it does without too much of a sales pitch.