For other kinds of jobs, allowing concurrent executions of multiple builds may be problematic, for example if it assumes a monopoly on a certain resource, like database, or for jobs where you use Jenkins as a cron replacement.
If you use a custom workspace and enable this option, all your builds will run on the same workspace, thus unless a care is taken by your side, it'll likely to collide with each other. Otherwise, even when they are run on the same node, Jenkins will use different workspaces to keep them isolated.
When Jenkins creates different workspaces for isolation, Jenkins appends "@num" to the workspace directory name, e.g. "@2". The separator "@" can be configured by setting the system property "hudson.slaves.WorkspaceList" to the desired separator string on the Jenkins command line. E.g. "-Dhudson.slaves.WorkspaceList=-" will use a dash as separator.