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 realizing 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.
