This workflow task permits you to add or delete tags on the object being saved. Either associated with the current user doing the operation or massively to all users.
This is very useful for filtering and segmenting your clients that are coming in through the webform, for example.
The workflow task in itself is simple; you select:
the action to do: add or delete
for whom to do it: all users or only current user
list of space-separated tags to work with