empty.pd is a template which I use for all of my new projects.
Copy empty.pd and load it, or copy the DSP patch into your project.
When you open PureData, DSP and Audio are disable by default. So you need to enable them manually. It is tedious to enable DSP each time you open your patches, so it is usefult to enable them automatically on startup.
Thats what the DSP patch in the empty.pd does.