So that you don't have find and clear filters one by one.
There's no visual aid that indicates that you have applied a filter to slicer. If you select a value in a slicer and the scroll so that item is not visible in the list, there is absolutely nothing that indicates that you have applied a filter. If you have many slicers on a report you'll have to go through all and clear them to be sure you've removed all filters.
