Difference between system app and 3rd party apps?

I was reading about best system optimisation app for android, and this thought came into my mind as what's the difference if you optimise through a 3rd party app or the system app. So let's say I am using X app (3rd Part) and I clean the space other app are using. It shows me how much memory it was using and shows me available space that I get after performing that action. Now if use the android system app to the same action, it shows some that there was some space that was realised hence allowing me to optimise . If both do the same whats the difference ?? In using 3rd party app and System app. !!! Confused