Generally large number of users log into SAP sytem. The dialog work processes (wp) available to the users are limited due to performance & system limits. The objective of a dialog work process is to handle the user requests quickly and display the results. Dialog work process also takes large resources relatively to background processes for display purposes. So it is not wise to have a dialog work process running for each user logged in.
Related: Work Process: Types in SAP
A dialog work process is not assigned to a single user for longer time. When a dialog user fires a request, the dispatcher assigns a dialog WP to handle the request. When the request is processed, it relieves the work process from that user so that other users can use it. This helps in handling requests from a large no of users with small no of dialog work process and optimum use of resources.