You see that big circle that counts down your current run/rest time remaining?
Well, if you click on it and you have the Free version you get a pop-up that tells you at least one of the perks of the Pro version.
If you have the Pro version, you actually get to see these perks. The perks are, upon clicking the circle, it will display your distance run, upon clicking the circle again, it will display your calories burned, and finally by clicking on the circle again, it will revert back to displaying the time remaining. (There may be more perks but this is all the pop-up tells me.)
I use a Samsung Galaxy S5 which came with the S Health app which already provides this information when running, however, I downloaded C25K to be more strategic about my training and I have tried to use both apps together and for some reason, S Health still provides my distance run so far (as well as current speed/pace) but it no longer provides my cadence (steps per minute) or calories burned, but the Google Fit app still reports these values.