NEGATION is comparable to the SQL NOT EXISTS as sometimes one wants information from the Knowledge Base or Database where some specific Pattern does not exists or one variables will not be fulfilled or can not be found. For example just give me all persons but only those that do not have provided a date, i.e. all without the date.
Thus one has the possibility to formulate some negation, e.g. to select all Person that do not have the specific Property that one defines.
On the other hand one can also select only those if one is referring to only those Variables that are fulfilled, i.e. really bound. However, this can be done in a much easier way.