When this option is selected, the credentials selection drop down will not provide the empty selection as one of the options. This will not prevent a build without a value if there are no credentials available, for example if the job does not have access to any credentials of the correct type or there is no default value and the user starting the build either does not have any credentials of the correct type in their personal credentials store or they do not have permissions on the job to use credentials from their personal store.