Let's say you want to fly to Tamworth. You can't enter via the overlying Class C, you would have to go via Duri Gap and enter the CTR directly. Same for Sydney, you could land at Kingsford Smith, if you entered directly from Class G, say, around Parramatta - you couldn't enter by coming in via the Class C step at Brooklyn Bridge at 3,000.
Conversely, if you held a CTA but not an aerodrome endo, then you could fly through overlying Class C at Tamworth, but not land there, or try your luck at a VFR transit at 8,500 through the Sydney basin in Class C, but not land at Bankstown, Camden or Sydney...