This is a question I know I asked myself when I first got an Apollo interface. Then after learning the answer I was quite gutted I had only had bought a quad. Bare in mind this going back quite a while before octo's were even thought of.
From what I gather the reason DSP is used even if you don't have any plugins active in console or in your DAW is because UAD's console software requires some DSP to run. You would think though that UAD would used a separate DSP chip or some sort of internal working to run the console software instead of using your precious DSP chips. Which lets face it, if you only have a duo interface they are running out quick.
If however you are experiencing unusually high amounts of DSP usage (pictured below) there is a quick fix. Drew at Universal Audio said in a recent Facebook post.
The built in pres and the connections themselves between the Apollo and Console along with delay comp, alt monitors, etc use DSP. If you press Control-Option-Command-Q, you'll quit Console and relinquish all DSP. Then relaunch Console and that will show you your minimum DSP usage for your setup.