Job foo runs once and creates a workspace named "foo".
After renaming foo to foofoo the workspace "foo" still exists if it wasn't deleted manually.
If foofoo runs now it creates it's own workspace.
The problem by having multiple jobs with huge workspaces is the disk space. In addition to that it's not nice to have some dead folders lying around on your file-system.
If job foo is executed in parallel it creates multiple workspaces.
This is fine as long as you're not using the "Wipe Out Current Workspace" button.
If so the only workspace that is getting deleted is foo but not the additional ones.
Example before wiping the workspace:
Example after wiping the workspace: