Bottomless (or 'naked') portafilters allow you to see the espresso as it exits the basket. This allows you to immediately identify any faults in your dosing or tamping technique. For example, you may be tamping slightly unevenly without realising it; with a bottomless portafilter you will see espresso leaving one side of the basket before the other side instead of it appearing evenly, something which would be impossible to gauge with a standard portafilter.

A number of espresso enthusiasts affirm the curious claim that the espresso from a bottomless portafilter also has a fuller flavour than that from a regular portafilter.
This is believed to be a result of less contamination from dirty surfaces over which the espresso would normally pass in a regular portafilter or from the espresso being affected by temperature differences in the portafilter.
We'll let you decide on that one, but taste benefit or not the bottomless portafilter is a must-have for any serious Breville BES920 series owner, including the Dual Boiler, Oracle Touch, the Oracle.