The tooltip on the Job/Workspace permission in the authorization configuration matrix after saying what the permission really does suggests that "if you don't want an user to access the source code, you can do so by revoking this permission".

Unfortunately the workspace is often only one of many ways to access the source code via Jenkins, which makes the suggestion rather misleading. Eg. for maven projects the archived source artifacts or the source xref report in the archived maven-generated site, both of which are accessible without the 'workspace' permission, give access to the sources.